This case was transferred to this Court under Rule 83.06 after opinion by the Missouri Court of Appeals, Eastern District. The single issue upon which transfer was granted is whether it was proper for plaintiff to comment during closing argument on defendant’s failure to call as a witness one of defendant’s employees. That issue is addressed in Part IV of this opinion, which borrows liberally from the opinion of Judge Gunn for the court of appeals. The remainder of Judge Gunn’s opinion, which sets forth the facts of the case and addresses appellant’s remaining contentions, is incorporated into and made a part of this opinion in Parts I, II, and III without use of quotation marks.
*788I
Plaintiff-respondent brought an action under theory of res ipsa loquitur against defendant-appellant Supreme Express & Transfer Company (Supreme) for personal injuries incurred when the forklift truck he was operating fell as Supreme’s trailer rolled away from the loading dock. Supreme appeals the jury verdict in favor of plaintiff for $13,000, raising the following points: (1) that plaintiff was contributorily negligent as a matter of law; (2) that plaintiff’s res ipsa loquitur instruction failed to embrace all the elements under the theory of the case; (3) that plaintiff, over objection, made erroneous prejudicial comment on defendant's failure to call a witness equally available to both parties. On this latter contention we find merit and are compelled to reverse and remand for a new trial.
Plaintiff was employed as a forklift operator by the Purex Corporation in St. Louis. Somewhere between midnight and 8:00 a.m. prior to the occurrence resulting in plaintiff’s injury, defendant’s driver had spotted a trailer at the Purex loading dock. The driver testified that to prevent the trailer’s movement, he set the air brakes for the trailer wheels, lowered its landing gear and blocked one of the dual wheels with a 4 X 4 wooden block. He then disengaged the tractor from the trailer. At approximately 9:30 in the morning, as plaintiff was driving his forklift onto the trailer with a load, the trailer slipped away from the loading dock. The forklift with plaintiff aboard dropped to the ground, landing on its wheels. Plaintiff sustained injury to his back as a result of the accident. The post-accident investigation disclosed that there was no wheel chock placed to prevent the trailer from rolling.
II
A Purex work safety rule in effect at the time of the accident provided that forklift operators were to look to determine if trailer wheels were chocked. Plaintiff testified that he did glance at the trailer to see if chocks were in place but that his vision was blocked and he could not see the wheels. He contends that he had been directed to fill a rush order which he considered had prevented him from taking time to step off his forklift and walk to a point where he could see whether chocks were in place. Defendant argues that violation of the work rule rendered plaintiff contributorily negligent as a matter of law; hence, the first point on appeal.
It is true, as defendant asserts, that contributory negligence can be a defense in a case brought under res ipsa loqui-tur fashion. Wissman v. Wissman, 575 S.W.2d 239, 242 (Mo.App.1978). But the issue of contributory negligence is ordinarily a jury question, becoming a matter of law only where reasonable minds cannot differ as to the plaintiff’s negligence. Mitchell v. Buchheit, 559 S.W.2d 528, 530 (Mo. bane 1977); Groppel Co. v. United States Gypsum Co., 616 S.W.2d 49, 62 (Mo. App.1981); Jenkins v. Jordan, 593 S.W.2d 236, 239 (Mo.App.1979). And although everyone is required to make ordinary use of his facilities to observe and avoid danger, whether ordinary care has not been exercised to raise the issue of contributory negligence depends on the particular facts of each case. Wissman v. Wissman, 575 S.W.2d at 240-41; Miller v. Sabinske, 322 S.W.2d 941, 946 (Mo.App.1959). The partic ular facts of this case lend themselves to a jury issue as to whether plaintiff was con-tributorily negligent in his actions in attempting to determine whether the trailer’s wheels had been blocked — an issue resolved against defendant.
Ill
Defendant alleges that the plaintiff’s verdict director1 is erroneous for failing to *789include the required finding that it exerted control over the parking and stabilization of the trailer.
An essential element to the application of res ipsa loquitur is that the defendant has management and control of the instrumentality involved at the time the negligence occurs. St. Joseph Light & Power Co. v. Kaw Valley Tunneling, Inc., 589 S.W.2d 260, 276 (Mo. banc 1979); Wagstaff v. City of Maplewood, 615 S.W.2d 608, 612 (Mo.App.1981); Davis v. Jackson, 604 S.W.2d 610, 612 n. 2 (Mo.App.1980). MAI 31.02(3), after which plaintiff’s verdict director was modeled, states:
Your verdict must be for plaintiff if you believe:
First, defendant (here describe defendant’s control, right to control, or management of the instrumentality involved)
Inserted within the parenthetical phrase relating to control was the following: “Defendant’s driver parked an empty trailer No. 93, at a loading dock .... ” It was undisputed at trial that defendant’s driver had the duty to park the trailer and immobilize it. It was also undisputed that the defendant’s driver was the only person who attempted to secure the trailer from rolling. There was no evidence that either the brake lines, landing gear or chocks were tampered with by anyone after the trailer was parked or that defendant had previous problems with anyone tampering with its vehicles on Purex’s lot.2 Though the instruction could more properly refer expressly to the defendant’s right of control, see, e.g., the instructions in Niman v. Plaza House, Inc., 471 S.W.2d 207, 212 (Mo. banc 1971), in light of the undisputed evidence, the instruction’s failure to refer to the defendant’s right of control was not prejudicial.3
IV
The remaining question is whether it was proper for plaintiff to comment during closing argument on defendant’s failure to call as a witness one of defendant’s employees, truck driver Clyde Riggs. Defendant argues that the comment was improper because the witness was equally available to both parties. Plaintiff claims the argument was proper because it was made in retaliation to an earlier argument made by defendant.
Defendant’s driver Norman Caldwell, who had parked the trailer at the Purex Corporation loading dock before his shift ended at 8 a.m. the morning of the accident, testified that he was “most positive” that he properly chocked the trailer wheels with a wooden block four inches square before he unhitched the tractor from the trailer. From this defendant, who conceded during summation that the trailer wheels were not chocked at the time of the accident, argued that the block might have been removed for use on another trailer. It is to this argument that plaintiff claims his argument retaliated.
The argument in question is as follows:
[PLAINTIFF’S ATTORNEY]: They want to tell you how fair they are. They want to tell you, “Mr. Leehy, go home.
*790Never mind your back. Never mind your problem.”
Where was the driver? He’s out there. He’s the guy that brought the tractor there. Try to throw the blame, is that being fair with you people?
Number two, the driver who was on duty out there between eight and four, Riggs. Was Mr. Riggs put on the stand? He’s still working out there. You want to talk about being fair? Mr. Leehy changing his testimony? You know what he’s trying to do in this case, don’t you?
[DEFENDANT’S ATTORNEY]: Again, Your Honor, I want to [object to] that characterization. He took Mr. Riggs’ deposition and the witness is equally available to him.
THE COURT: Your objection is overruled.
[PLAINTIFF’S ATTORNEY]: You know, let’s get a few facts. I think this jury knows what’s going on in this case. You know, when you start thinking about it, you know, you bring in this and you bring in that. He’s got two opposite contradictions: Somebody must have stole the block. You know, that trailer could have rolled.
Plaintiff’s argument was not proper as retaliation. The tenor of plaintiff’s argument is that defendant deliberately attempted to deceive the jury by concealing material evidence and laying the blame elsewhere. The argument in no real sense responds to defendant’s argument. Earlier in his argument plaintiff had made the jury aware that “[t]here’s no evidence” that the block was borrowed or stolen. The further argument at issue here was unnecessary to make that point and cannot be said to constitute proper retaliation.
Defendant’s contention that the argument was improper because Riggs was equally available as a witness to both parties is meritorious.
The failure of a party to call a witness having knowledge of facts and circumstances vital to the case generally raises a presumption that the testimony would be unfavorable to the party failing to proffer it. Block v. Rackers, 256 S.W.2d 760, 764 (Mo.1953). It is improper, however, for a party to argue the negative inference resulting from his opponent’s failure to produce such a witness if the witness is equally available to both parties. Hill v. Boles, 583 S.W.2d 141, 145 (Mo. banc 1979). The trial court’s failure to sustain an objection to such an improper argument constitutes prejudicial error. Id.
Whether a witness is to be considered equally available to the parties on both sides of a lawsuit depends upon several factors. Included among those factors are (1) “one party’s superior means of knowledge of the existence and identity of the witness”; (2) “the nature of the testimony that the witness would be expected to give in the light of his previous statements or declarations, if any, about the facts of the case”; and (3) “the relationship borne by the witness to a particular party as the same would reasonably be expected to affect his personal interest in the outcome of the litigation and make it natural that he would be expected to testify in favor of the one party against the other.” Id. at 145 — 46. While it would be needlessly broad to hold, as defendant urges, that Riggs was equally available to plaintiff simply because plaintiff took Riggs’ deposition, application of the foregoing factors to the facts of this case makes it clear that Riggs was indeed equally available to plaintiff and defendant.4
*791The first factor points toward equal availability. The fact that plaintiff deposed Riggs demonstrates fully that plaintiff knew both of the existence and the identity of this potential witness. Although Riggs was not assigned to the Purex Corporation loading dock on a regular basis, he worked there as a replacement the week the accident occurred and had worked there during the entire winter of 1975 — all during the time period that plaintiff was employed at Purex as a forklift operator. Moreover, even if plaintiff did not know Riggs personally, defendant’s business records indicate that Riggs was assigned to the Purex Corporation site at the time of the accident, and those records could have been obtained easily through normal discovery procedures. It cannot be said, therefore, that defendant had a “superior means of knowledge of the existence and identity” of Riggs.
The second and third factors are related. It might reasonably be expected that Riggs would testify in favor of defendant, his employer. Riggs’ deposition transcript, however, demonstrates otherwise. Nothing in any of Riggs’ answers to plaintiff’s questions indicates that Riggs was in any way evasive or uncooperative. The fact is that Riggs could not testify persuasively for either side because he knew very little about the incident. He was on duty at the time of the accident, but he learned about the accident from a dock employee after he returned from making a delivery. Any testimony he might have given relating directs ly to the accident would have been hearsay. Neither could he have testified whether the trailer wheels were chocked when the trailer was parked at the loading dock, because he was not on duty at the time the trailer was parked. In short, Riggs’ testimony would have been virtually meaningless.5
The record amply demonstrates that plaintiff’s argument was improper because Riggs was equally available as a witness to plaintiff. The argument cannot be sustained on the ground that it constituted proper retaliation. It therefore should have been excluded. The trial court committed prejudicial error by overruling defendant’s objection and allowing plaintiff to proceed with the argument. Id. at 145. The judgment of the trial court is reversed, and the case is remanded for a new trial.
WELLIVER, HIGGINS and DONNELLY, JJ., and SEILER, Senior Justice, concur. RENDLEN, C.J., dissents in separate opinion filed. *792MORGAN, Senior Justice, dissents and concurs in separate dissenting opinion of RENDLEN, C.J. GUNN, BILLINGS and BLACKMAR, JJ., not participating because not members of the Court when cause was submitted.. Plaintiff’s verdict director provides:
Your verdict must be for plaintiff if you believe:
First, defendant’s driver parked an empty trailer, No. 93, at a loading dock, and
Second, plaintiff was driving his forklift truck into said trailer, when suddenly and *789unexpectedly said trailer rolled away from said dock causing plaintiffs forklift truck to fall to the ground, and
Third, from the facts in evidence and the reasonable inferences therefrom, you find such occurrence was the direct result of defendant’s negligence, and
Fourth, as a direct result of such negligence, plaintiff sustained damage.
Unless you believe, plaintiff is not entitled to recover by reason of Instruction No. 5.
. Defendant attempted to bring out in cross-examination that drivers of other dray companies would remove blocks from other trailers for their own use. Objection to such relevant testimony which could conceivably remove the control issue and, hence, res ipsa loquitur was sustained. But no point is made of this factor on appeal.
. The given instruction is similar to that in City of Kennett v. Akers, 564 S.W.2d 41, 44 (Mo. banc 1978), in which plaintiffs verdict director stated: “Your verdict must be for plaintiff if you believe: First, defendant allowed the antennae tower to fall into the electric power line .. .. ” Plaintiffs instruction made no express reference to defendant’s control. There was no direct attack upon the instruction for that reason, but it was noted that exclusive control was in the defendant. Id. at 46.
. The balancing test enunciated in Boles forecloses an extreme holding either that an employee is necessarily more available to his employer simply because of his status as an employee or that a witness associated with one party is necessarily equally available to the other if the other has deposed him. Some pre-Boles cases suggest that the existence of an employer-employee relationship automatically makes the employee more available to the employer. See, e.g., Zipp v. Gasen’s Drug Stores, Inc., 449 S.W.2d 612, 620 (Mo. 1970); Wehrkamp v. Watkins Motor Lines, Inc., 436 S.W.2d 698, 716 (Mo. 1969); Duboise v. Railway Express Agency, Inc., 409 S.W.2d 108, 113 (Mo. 1966). See also Goodman v. Firmin Desioge Hosp., 540 S.W.2d 907, 913 (Mo.App.1976). Similarly, other pre-Boles cases suggest that *791when one party takes the deposition of a witness associated with the opposing party, the witness automatically becomes equally available to both parties. See, e.g., Bean v. Riddle, 423 S.W.2d 709, 721 (Mo.1968); Midwest Library Serv., Inc. v. Structural Syss., Inc., 566 S.W.2d 249, 252 (Mo.App.1978). Cf. Lix v. Gastian, 287 S.W.2d 354, 357 (Mo.App.1956) (availability of transcript from first trial made witness who testified at first trial equally available at second trial). The philosophies underlying these two lines of cases clash in this case, in which the witness in question is an employee of one party and has been deposed by the other. This case thus highlights the impracticality of inflexible rules. It would be better to say that, as with the party-physician relationship, see Boles, 583 S.W.2d at 146, a presumption arises regarding the availability of the witness and that “in a proper case this presumption may be held inapplicable by reason of the circumstances shown in evidence,” id. Compare Lewis v. Bucyrus-Erie, Inc., 622 S.W.2d 920, 927 n. 6 (Mo. banc 1981). Application of the balancing test mandated by Boles would operate either to solidify or to dispel the presumption.
. The dissent argues that Riggs did indeed testify to a number of material issues. The dissent, however, misconceives the nature of Riggs’ testimony. The testimony about whether the trailer involved in this case would roll if the brakes did not work and the wheels were not chocked was in response to a hypothetical question posed by plaintiff. The claimed admission that “sometimes they [the brakes] do not work” because “the air could leak off’ was in response to a question relating to Riggs’ experience with trailers in general and was not in response to a question regarding the trailer involved here.
Oddly enough, the dissent’s characterization of Riggs’ testimony supports the conclusion that as a witness Riggs was equally available to plaintiff. If the tenor of the testimony were as the dissent characterizes it, the testimony doubtless would have favored plaintiff rather than defendant, and it would seem that plaintiff would have called Riggs as a witness.