delivered the opinion of the court.
Larry Donell Robinson, defendant, was indicted in two counts for the rape and malicious wounding of a young woman in the City of Alexandria. A jury found Robinson guilty of both offenses, and fixed his punishment at thirty-five years in the State Penitentiary for rape and fifteen years for malicious wounding. The trial court sentenced Robinson in accordance with the jury verdict. We granted him a writ of error to that judgment.
Upon assurance of the Commonwealth’s Attorney that the con*137tinuity of possession of certain exhibits, including a pair of lady’s panties, a blouse and some pubic hair taken from the victim, would be shown, the court permitted P. R. Bidez and M. S. Clark, FBI special agents, to express their opinions resulting from their examination of these exhibits. Bidez testified that the above mentioned exhibits were delivered to him by Douglas M. Thompson, a member of the Alexandria Police Department. The agents’ examinations revealed that semen stains and blood stains were on the panties; that on the blouse were wool fibers similar to wool fibers in the sweater Robinson was wearing when arrested soon after the crimes were committed; and that the pubic hair taken from the victim had fifteen similar characteristics to a pubic hair removed from Robinson’s undershorts. These exhibits were returned to Officer Thompson by Bidez after the examinations were made.
Officer Thompson, who was in charge of the Alexandria police property room, testified that he received from a nurse at the hospital the victim’s panties and an envelope containing some pubic hair. He also stated that he received the victim’s blouse from Sergeant Pendergraph, another officer of the Alexandria Police Department. These items, the testimony shows, were properly handled by Officer Thompson. Each exhibit was sealed in a bag and kept out of contact with other exhibits the police had obtained through their investigation.
The victim identified the blouse and the panties as garments she was Wearing when attacked. She also testified that pubic hair was taken from her at the hospital and placed in an envelope. She identified an envelope as being like the one used.
However, the Commonwealth’s evidence does not show what was done with these exhibits from the time they were taken from the victim to the time they, were delivered to Officer Thompson. The nurse, who delivered the panties and pubic hair to Officer Thompson, did not testify. Nor is there any testimony as to how Sergeant Pendergraph came into possession of the blouse or what he did with the blouse before it was delivered to Officer Thompson.
In this appeal the critical question is whether the trial court erred in admitting into evidence the panties, blouse and pubic hair, and the opinion testimony of the FBI agents relative to these exhibits.
Counsel for Robinson asserts that the failure of the Commonwealth to call as a witness the unidentified nurse caused a break in the chain of possession of the panties and the pubic hair. Likewise, the failure of Sergeant Pendergraph to testify concerning the blouse *138the victim was wearing when attacked caused a break in the chain of possession of it. Because an unbroken chain of possession was not established, it is argued, the exhibits and the opinions of the FBI agents concerning their analyses of them were not admissible.
In Rodgers v. Commonwealth, 197 Va. 527, 90 S.E.2d 257 (1955) the results of a blood analysis had been admitted into evidence without sufficient identification of the blood as that of the defendant. We said in that case “[sjuch an analysis is important evidence in a trial of this sort, and care must be exercised to establish the essential links in th'e chain of evidence relied on to identify the blood analyzed as being the blood taken”. 197 Va. at 531, 90 S.E.2d at 259. The basic rule for admitting demonstrative evidence is that the burden is upon the party offering the evidence to show with reasonable certainty that there has been no alteration or substitution of it. But the burden is not absolute that “all possibility of tampering” be eliminated. People v. Riser, 47 Cal. 2d 566, 580-81, 305 P.2d 1, 10 (1957). However, “ [t] he requirement of reasonable certainty is not met when some vital link in the chain of possession is not accounted for, because then it is as likely as not that the evidence analyzed was not the evidence originally received.” (Italics supplied.)
In the case at bar the Commonwealth failed to establish a chain of possession of the panti'es, blouse and pubic hair. It is not reasonably certain from the testimony presented that these exhibits were in the same condition when analyzed as they were when taken from the victim. Furthermore, it was not shown that the pubic hair taken from the victim was the same public hair that was analyzed.
A vital link in the chain of possession of these 'exhibits was the treatment they received from the time they were taken from the victim until delivered to Officer Thompson. Yet, the Commonwealth failed to establish this vital link in the chain of possession. We cannot assume that these exhibits were properly handled. Without an unbroken chain of possession of the panties, blouse and pubic hair, they were not admissible as evidence insofar as they supplied a basis for the opinion testimony of the FBI agents, who had examined them. Thus, th'e opinions of the FBI agents were also not admissible.
It is true that the blouse and panties were identified at the trial by the victim as a part of the clothes she was wearing when attacked. The Commonwealth contends that they were properly admitted because the victim identified them. If they had been admitted only to establish what the victim was wearing when attacked, then we would agree with the Commonwealth’s position. But the blouse and *139the panties were also admitted to supply a basis for the opinion testimony of the FBI agents. The mere fact the blouse and the panties were identified did not prove the chain of possession necessary to validate the FBI analysis of them.
The error in admitting the exhibits as a basis for the testimony of the FBI agents was not harmless. In his closing argument before the jury, th'e Commonwealth’s Attorney stressed the importance of this evidence. He referred to it as “scientific proof to back up that young lady’s story that this man is the one.”
For the reasons stated, the judgment appealed from is reversed and the case remanded for a new trial.
Reversed and remanded.