A jury in the Superior Court convicted the defendant of murder in the first degree on theories of deliberate *481premeditation and extreme atrocity or cruelty. Represented by a new attorney, the defendant filed a motion for a new trial. The judge assigned to the motion (the trial judge having retired) took no action on it, leaving the defendant’s claims for us to decide on this direct appeal. See Commonwealth v. Erdely, 430 Mass. 149, 150 (1999). We conclude that portions of the jury instructions concerning voluntary manslaughter were erroneous and posed a substantial likelihood of a miscarriage of justice. Consequently, we reverse the defendant’s conviction of murder in the first degree, set aside the jury verdict, and remand the case for a new trial.
At trial, there was no dispute that the defendant killed the victim. His defense was that he was not guilty of murder because he acted in self-defense. Trial counsel argued to the jury (for their consideration as an alternative verdict) that the defendant was guilty only of voluntary manslaughter because he had acted on reasonable provocation or had used excessive force in self-defense. Trial counsel also urged the jury to consider the effect that drug and alcohol consumption might have had on the defendant. The judge instructed the jury that they could consider the effect, if any, that drug and alcohol consumption might have had on the defendant’s ability deliberately to premeditate and to form the requisite intent to kill. Further, as the framework of the case required, the judge instructed the jury on murder in the first degree on the theories of deliberate premeditation and extreme atrocity or cruelty, murder in the second degree, voluntary manslaughter, and involuntary manslaughter.
The evidence, considered for purposes of the issues on this appeal in the light most favorable to the defendant, see Commonwealth v. Carlino, 429 Mass. 692, 693 (1999), would have warranted the jury in finding the following. On February 24, 1995, after drinking eight or nine beers within three hours, the defendant “got the urge to do some cocaine.” He left his apartment and purchased some cocaine and returned home. He divided the crack cocaine into six “hits.” Shortly after 11 p.m., the defendant smoked the first “hit.” He then called the victim, with whom he had had an eleven-month relationship and with whom he had previously smoked cocaine. After speaking with *482the victim, the defendant left one “hit” of cocaine at his apartment and took the remaining four “hits” to the victim’s apartment.
At the victim’s apartment, the defendant and the victim each smoked two “hits” and drank beer. A little after 1 a.m., on February 25, 1995, the victim told the defendant that she would telephone her dealer to get some more cocaine. When it became apparent that the victim was not going to, the defendant went home. There he smoked his last “hit” of cocaine, and drank another beer. Shortly after 2 a.m., the victim telephoned the defendant, inviting him back to her apartment. He returned and brought a green duffel bag with him. The defendant and the victim drank beer until the defendant got tired. The victim asked the defendant to stay the night, and he did. (The defendant and the victim did not engage in sexual relations.)
When he awoke, as he was getting ready to leave, the defendant recalled that the victim had taken forty-five cents from him the previous night, and asked her to return the money. The victim replied that she did not know where the money was. The defendant complained that every time he visited her, he would discover something missing from his pockets. The defendant again asked for his money back. The victim said that she needed to take a taxicab to her credit union. The defendant told her that he did not think it was “right” for her to take his forty-five cents to “pedal [her] fat ass around in a taxi.” In response, the victim hit the defendant hard on the side of his face and his glasses “went flying.” The defendant became “pissed . . . off” and punched the victim on the back. A fistfight ensued. During the fight, the defendant tried to restrain the victim. She bit his left thumb at his knuckle, then ran to the kitchen and grabbed a knife from the counter.
The defendant was aware that the victim could be violent.1 He pulled out a knife that he carried “for protection” from his pocket. Panicking, because the victim was blocking the only way out of the apartment and because he had a medical condi*483tion that impaired his mobility,2 the defendant moved toward the victim and the doorway. They grabbed each other’s right hands, which held their knives, and began wrestling across the apartment while “trying to hurt each other.”
At one point, the defendant cut the victim’s cheek, which “freaked” him, causing him to drop his knife. He yelled, “You’re cut, you’re cut, drop your knife, drop your knife.” The victim did not comply with his request. Each continued to hold onto each other’s hand. The knife that the victim held was coming close to the defendant’s face. He was in a panic and felt that he was going to die. The defendant noticed a large knife near the sink and grabbed it. The defendant could “actually recall” only stabbing the victim once, stabbing her in the neck. He conceded, however, that he could have stabbed her more than once with the large knife, and more than once with a small knife.3
With the large knife in her neck, the victim stopped fighting. The defendant eased her to the floor, then removed the knife. He panicked when he realized what he had done. He rinsed the knives in the kitchen sink and then put them in his bag. There was blood “everywhere,” and the defendant was scared and in shock. He was staring at the victim’s body when police officers entered the apartment and arrested him. The defendant had several scratches and scrapes on his face and hands that were not present before his altercation with the victim.
1. We need not address all of the defendant’s challenges to the jury instructions because we conclude that the errors in the judge’s instructions on voluntary manslaughter require reversal. The defendant’s trial counsel did not object to the instructions on the grounds the defendant asserts now.4 We thus review for a substantial likelihood of a miscarriage of justice. See Com*484monwealth v. Carlino, supra at 695-696; Commonwealth v. Rosa, 422 Mass. 18, 29-30 (1996).
(a) The Commonwealth concedes error in the portion of the judge’s instruction on voluntary manslaughter which states that “to prove the defendant guilty of voluntary manslaughter, the Commonwealth must prove . . . beyond a reasonable doubt . . . that the defendant injured [the victim] as a result of sudden combat or in the heat of passion.” See Commonwealth v. Acevedo, 427 Mass. 714, 716 (1998). See also Commonwealth v. Boucher, 403 Mass. 659, 661 (1989). In the Acevedo decision, we concluded that a similar instruction was erroneous because it “incorrectly told the jury that malice is negated by provocation only if provocation is proved beyond a reasonable doubt. The correct rule is that, where the evidence raises the possibility that the defendant may have acted on reasonable provocation, the Commonwealth must prove, and the jury must find, beyond a reasonable doubt that the defendant did not act on reasonable provocation.” Id. We explained that, “[t]he difference between proof beyond a reasonable doubt that a defendant acted with reasonable provocation and proof beyond a reasonable doubt that a defendant did not act with reasonable provocation is substantial.” Id. at 717. The Commonwealth claims that this error did not create a substantial likelihood of a miscarriage of justice because the judge’s “instructions read as a whole informed the jury that the Commonwealth bore the burden of disproving provocation and self-defense beyond a reasonable doubt.” We disagree.
Here, contrary to the Commonwealth’s assertion, the judge only once correctly instructed the jury on the Commonwealth’s burden of proof on provocation.5 Although the judge’s correct instruction on that subject followed his erroneous instruction, the correct instruction did not make it “clear to the jury” that it “carried] more weight than the . . . incorrect one[].” Commonwealth v. Fickling, 434 Mass. 9, 20 (2001). Cf. id., quoting Commonwealth v. Acevedo, supra at 111 (concluding that erroneous instructions were counterbalanced by correct instruc*485tions so that we could not conclude that “the center of gravity of the provocation instructions was strongly on the side of misstatement”). The judge neither acknowledged nor pointed out to the jury in any meaningful manner that his first instruction on the issue had been erroneous. Aggravation of the error occurred when the judge told the jury, “You can’t accept one premise, as opposed to another premise as being a superior instruction, because all. of these instructions have equal weight.”6
The judge’s reminder to the jury that “the defendant does not have to prove anything” does not aid the Commonwealth. The judge gave that instruction not in the context of the provocation instruction, but rather, in his instruction concerning self-defense, which he discussed before even mentioning the separate mitigating circumstance of provocation. That the judge gave some correct instructions to the jury on self-defense is not consequential. As noted by the defendant, “[a] jury required to make a consistent whole of inconsistent instructions might reasonably find that the Commonwealth bore the burden of disproving facts mandating acquittal, but once acquittal was not an option the defendant had the burden ... of proving murder was mitigated [when there is sufficient evidence of provocation], to manslaughter.” In addition, the following cases cited by the Commonwealth are distinguishable. See Commonwealth v. Ni-emic, 427 Mass. 718, 721-722 (1998) (although judge correctly instructed that Commonwealth bore burden of disproving heat *486of passion, after making Acevedo error, provocation was not “live” issue in case and judge, apparently, did not instruct jury that all instructions carried equal weight); Commonwealth v. Hung Tan Vo, 427 Mass. 464, 471-472 (1998) (involving error in instructions on deliberate premeditation); Commonwealth v. Torres, 420 Mass. 479, 488 n.8 (1995) (incorrect instruction on provocation sandwiched between two correct instructions, and judge emphatically and repeatedly stated that only Commonwealth, and never defendant, bore any burden of proof).
Finally, we reject the Commonwealth’s contention, raised for the first time on appeal, that provocation was not a “live” issue at trial. While the defendant’s principal defense was self-defense, there was sufficient evidence that obligated the judge to instruct the jury correctly on provocation. For example, there was evidence (which we must accept as true for purposes of this appeal) that the victim was the first to strike, that she previously had been violent toward the defendant, that no avenue of retreat was available to the defendant, and evidence that would permit the jury to find the whole episode stemmed from a volatile, unexpected confrontation. In his closing argument, the defendant’s trial counsel argued that, at best, the Commonwealth had proved manslaughter, not murder, because of the evidence of provocation. When weighed with the additional errors in the voluntary manslaughter instruction, which we next discuss, we conclude that there must be a new trial.7
(b) On the issue of voluntary manslaughter, the judge instructed the jury that “the factor that distinguishes voluntary manslaughter from murder is not the absence of intent but rather the absence of malice aforethought. That is the absence of a specific intent to kill or the absence of a specific intent to do great bodily harm.” Later, after setting forth the elements of voluntary and involuntary manslaughter, the judge stated, “If *487the Commonwealth has failed to prove an unlawful killing, an intentional killing, if they have not proven malice aforethought, you will then consider whether the Commonwealth has proven voluntary manslaughter.” These instructions are erroneous and misleading. They improperly suggest that the Commonwealth does not have to prove an essential element of voluntary manslaughter, namely, an intentional killing. Commonwealth v. Whitman, 430 Mass. 746, 753 (2000), quoting Commonwealth v. Squailia, 429 Mass. 101, 109 (1999) (voluntary manslaughter is “an intentional killing, which is mitigated by extenuating circumstances” [emphasis in original]). When viewed in conjunction with the other errors in the voluntary manslaughter instruction discussed above, the judge’s instruction potentially deprived the defendant of his mitigating defense.8
Our conclusion is buttressed by the fact that, in response to a question by the jury for redefinition of the elements of murder in the second degree and malice, the judge gave a compounding incorrect instruction on self-defense when, in answer to their question, he told the jury that, if the Commonwealth had not proved the lack of self-defense, they should “then consider whether the Commonwealth has proven [that] the defendant did not act with excessive use of self-defense.” The erroneous instructions (i) confused the jury on the role of provocation; (ii) conveyed the wrong burden of proof on key elements of voluntary manslaughter; and (iii) possibly led the jury to think that voluntary manslaughter could be considered as a verdict only if the Commonwealth failed to prove an intentional killing. These errors went directly to the important factor of malice, a matter that the jury’s question reflects was troubling to them. Of course, we do not know how the jury resolved the issues. But, in a case where voluntary manslaughter was a viable *488defense, we cannot tell whether the jury may have disregarded consideration of manslaughter because of the erroneous instructions and instead opted only to consider the higher verdicts. The jury’s question suggests, if nothing else, that they were concerned whether the Commonwealth had proved malice, and they were discussing the law applicable to a verdict less than murder in the first degree. Thus, we cannot fairly say that the errors did not give rise to a substantial likelihood of a miscarriage of justice.
2. We comment briefly on issues that may arise at any retrial. (a) At trial, the defendant testified that at the time of the killing, although he believed he stabbed the victim repeatedly, he only recalled stabbing the victim once in the neck. He explained that he “was just being carried along.” Considering this testimony, the judge did not err in refusing to instruct the jury that “any actions conducted by the defendant after the killing cannot be used in the determination of malice.” See Commonwealth v. Rice, 427 Mass. 203, 210 (1998).
(b) The defendant argues that, “because the police did not inform [him] at the time of his arrest that he could obtain an independent test for the presence of alcohol and drugs in his body, and made no such tests themselves,” the judge’s refusal to instruct the jury, pursuant to Commonwealth v. Bowden, 379 Mass. 472, 485-486 (1980), requires reversal. Trying to circumvent the rule that the decision to give a so-called “Bowden instruction” is a matter of discretion for the trial judge, see Commonwealth v. Rivera, 424 Mass. 266, 274 (1997), cert, denied, 525 U.S. 934 (1998), the defendant essentially argues that no such discretion existed in this case because the police failed to preserve all exculpatory evidence, namely, the drug and alcohol content of the defendant’s blood. The argument has no merit. While the prosecution remains obligated to disclose all exculpatory evidence in its possession, it is under no duty to gather evidence that may be potentially helpful to the defense. See Commonwealth v. Neal, 392 Mass. 1, 8-9 (1984).
(c) The defendant’s remaining challenges to the judge’s instructions to the jury need not be discussed. We presume that tie judge at any retrial will instruct the jury in accordance with *489our model jury instructions on homicide. See Model Jury Instructions on Homicide (1999).
3. We reverse the defendant’s conviction of murder in the first degree, set aside the jury’s verdict, and remand the case to the Superior Court for a new" trial.
So ordered.
On a prior occasion, the victim had cut the defendant’s ear with a blow from behind because he would not pay for their taxicab ride.
The defendant’s right hip was held together by surgical screws.
The medical examiner testified that the victim died from multiple stab and incisor wounds.
The lack of an objection is not remarkable. The principal infirmity in the instructions was not clearly identified until Commonwealth v. Acevedo, 427 Mass. 714, 716 (1998), which was decided after the defendant’s trial, although somewhat foreshadowed by dicta in Commonwealth v. Boucher, 403 Mass. 659, 661 (1989). The deficient instruction that we shall discuss shortly was in common use in the Superior Court at the time of the defendant’s trial.
The judge adequately stated, “If there is evidence of provocation, the Commonwealth has the burden of proving beyond a reasonable doubt that the defendant did not act in the heat of passion.”
Contrary to the dissent’s intimation, we have reviewed the jury instructions in their entirety. That review demonstrates that the judge did not adequately and clearly explain voluntary manslaughter to the jury. In the instructions there are contradictory statements concerning the Commonwealth’s burden of proof on provocation, as well as whether an intentional killing constitutes voluntary manslaughter. The dissent reasons that, with respect to the Acevedo error, because a correct instruction of the law followed an incorrect instruction, the incorrect instruction was “immediately corrected,” and that the Commonwealth’s burden of proof on provocation was “unequivocal.” Post at 491. This is not so. As we previously noted, the judge never acknowledged that his instructions on the issue had been flawed, and, thus, never supplanted the incorrect instruction with a correct one. To the contrary, the judge told the jury that his instructions carried equal weight. Although we, as Justices educated in the law, are able to discern which instruction was correct and which instruction was incorrect, it cannot be said that the jury could have done so on their own.
The Commonwealth also argues that provocation by means of sudden combat and self-defense are virtually indistinguishable. The argument suggests that errors in the provocation instructions are inconsequential, if the defendant claims' self-defense as incident to sudden combat. Our law has never considered the two issues as mutually exclusive. In a case like this, the defendant is entitled to correct instructions on both provocation and self-defense, and the jury are to have an opportunity to consider voluntary manslaughter on both theories.
The dissent unfairly charges the jury with knowing and understanding legal definitions, such as malice, and dismisses an incorrect instruction as a mere “slip of the tongue.” Post at 495. The cumulative effect of several errors in the instructions pose a substantial likelihood of a miscarriage of justice. In addition, the “failure of counsel” to have objected to any errors below does not necessarily “strongly suggest!] that it was not of consequence to those who heard it.” Id. The lack of an objection is, as has been stated, most likely explained by the fact that the Acevedo case had not yet been decided. See note 4, supra.