SFVs are exogenous retroviruses enzootic in both New and Old World primates (16) (17) (18) . Phylogenetic analyses of SFVs indicate a species-specific distribution of virus strains not unlike that of SIV among some African primate species (19) (20) (21) (22) (23) . Among captive primate populations, seroprevalence of antibodies to SFV may reach 100% in adults, with many animals seroconverting before the onset of sexual maturity (19, 24 ; J. Allan, unpub. data). Fewer data are available on the seroprevalence of antibodies to SFV among free-ranging populations of primates. SFV is present in highest concentrations in the saliva of infected laboratory macaques (Macaca mulatta and M. fascicularis) and African green monkeys (Cercopithecus aethiops), which suggests that bites, scratches, and mucosal splashes with saliva from primates are likely mechanisms of transmission (25) (26) (27) .