Simian immunodeficiency virus (SIV) is a Lentivirus that exhibits both morphological and genomic similarities to HIV. SIV has been isolated from sooty mangabeys in several primate colonies, and has been shown to be about 80% related to HTV-2. Since HIV-2 and sooty mangabeys are both indigenous to West Africa, isolation of a virus from sooty mangabeys in West Africa would be of great interest in determining the origins of these two viruses. We report here thin section transmission electron microscopy of a lentivirus from a pet sooty mangabey living in Liberia, West Africa.Peripheral blood mononuclear cells (PEMC) from a healthy pet sooty mangabey in Liberia were frozen in liquid nitrogen and shipped to the California Primate Research Center. The cells were thawed and stimulated for 72h with 0.5μg/ml Staphylococcal enterotoxin A in RPMI 1640 containing 10% fetal calf serum. 2×106PEMC were then co-cultivated with 2×106CEMxl74 cells and processed for EM.CEMxl74 cells co-cultivated with infected PBMC exhibited syncytial giant cells with patchy foamy cytoplasm (Fig 1). large numbers of virus particles were observed within these vacuoles (Fig 2).