Retrospective data indicate that two separate outbreaks of simian AIDS and associated lymphoma were caused by Simian Immunodeficiency Virus (SIVmac and SIVstm, respectively) in group-housed macaques at the California Regional Primate Research Center (CRPRC) in the early and mid-1970s. Because these epizootics were not then recognized as infectious in nature, surviving healthy SIV carriers were sent to other primate centers where they transmitted the viruses to resident macaques. The source of SIV at the CRPRC was by contact with co-housed seropositive sooty mangabeys. Spread of SIV via saliva and blood while fighting most likely accounted for these epizootics. Separate outbreaks of a somewhat different version of simian AIDS, caused by the simian Type D retrovirus (SRV-1), and spread from healthy carriers via saliva and blood also occurred in the late 1970s and early 1980s in group-housed macaques at the CRPRC. Initially, these SRV-1 outbreaks were also not recognized as infectious.