A virus neutralization assay was examined in this article that is essentially the same or quite similar to neutralization assays used in other SIV and HIV research laboratories. Serum samples from SIV-infected rhesus monkeys were heat inactivated and incubated with standard amounts of SIV; virus-serum mixtures were used to infect permissive cells and viral p27 gag protein production was monitored four days later with an antigen capture kit. The results of our control experiments indicate that sera from SIV-infected rhesus monkeys often contain high levels of anti-gag antibodies which interfere with the measurement of viral p27 protein by the antigen capture detection system. Thus, the presence of anti-gag antibodies precludes meaningful measurement of virus neutralizing activity by this commonly used procedure.