Polymerase chain reaction techniques were used to identify simian immunodeficiency virus (SIV) SIVsmm gag sequences in genomic DNA isolated from peripheral blood mononuclear cells from naturally infected asymptomatic seropositive and seronegative sooty mangabeys (Cercocebus atys) and from experimentally infected but asymptomatic rhesus macaques (Macaca mulatta). The results indicate that most if not all SIV-seronegative mangabeys from the colony at the Yerkes Primate Center are in fact infected with SIVsmm despite their lack of humoral immune response, confirming previous immunological and virological observations made by our laboratory. Sequence analysis of these particular gag fragments from the mangabey revealed an average of 88% nucleotide sequence homology but 97% amino acid identity with the previously published sequence of the SIVsmmH4 clone. The significance of this finding relative to the asymptomatic state of SIV-infected mangabeys and disease-susceptible SIV-infected rhesus macaques is discussed.