Rhesus macaque monkeys infected with the simian immunodeficiency virus develop a syndrome mimicking AIDS in humans. We have demonstrated previously that sera from individuals infected with human immunodeficiency virus type 1 inhibit the proliferation of lymphocytes from healthy noninfected subjects and that this phenomenon is associated with the development of clinical AIDS. We have also shown that sera from monkeys infected with SIV also have such inhibitors. In this body of work, we attempted to document the onset of these inhibitors in relation to the time of SIV infection. Twenty rhesus macaques were injected with one of two tissue strains of SIV or media. Blood was drawn on a set schedule and the serum samples frozen at -70 degrees C. The animals were monitored and observed for up to 42 weeks. All test animals were autopsied. Sera from all the draws were assayed against the same populations of human peripheral blood mononuclear cells in the same experiment using suboptimal amounts of phytohemagglutinin (PHA). Sera from those animals that subsequently developed SAIDS were more likely to demonstrate serum inhibition. This inhibition could be seen as early as 8-10 weeks after infection. By week 14, the assay could differentiate animals into SAIDS or healthy groups with a sensitivity of 67% and a specificity of 89%.