One of the theories explaining the origin of the AIDS epidemic hypothesized that an oral polio vaccine (OPV), used in the Belgian Congo in the late 1950s, was contaminated when produced in chimpanzee epithelial cell cultures. In the April 27 Science, Poinar et al. provide molecular evidence to refute this hypothesis (Science 2001, 292:743-744). They analysed DNA from 14 laboratory samples from the Wistar Institute in Philadelphia, in a blind study, to search for chimpanzee DNA material. The authors used real-time quantitative PCR analysis to determine the source origin of the cells used in vaccine production. They amplified a 141 bp fragment of the nuclear 28S rDNA gene and sequenced the PCR products. The sequences of all the OPV batch samples were identical and were the same as monkey DNA controls, but differed from chimpanzee DNA at two positions. A number of tests indicate that chimpanzee DNA would have been detected by this analysis, even if it represented just 0.01% of total DNA in the vaccine samples. The authors conclude with confidence that the source DNA in the OPV batches is derived from old-world monkeys and has no chimpanzee contamination.