Antibodies to proteins of Mason-Pfizer monkey virus (M-PMV) were screened in sera from 61 healthy donors living in Guinea-Bissau (4 HIV-1 and HIV-2 antibody positive HIV = human immunodeficiency virus), from 19 healthy French European blood donors, and from 9 French patients with induced immunodeficiency prior to bone marrow transplantation (all HIV-1 antibody negative). In 30 (49%) of the African sera tested, antibodies reacting against p27 and/or p14 were detected by western blot. Some of these cases were confirmed to be positive using radioimmuno precipitation assay. Only one serum from a French blood donor was detected by western blot to be slightly positive against p27 M-PMV. Thirty-two sera screened for M-PMV were also tested for squirrel monkey retrovirus by western blot. In eight (25%) sera antibodies at least against p36 were detected. Among squirrel monkey retrovirus positive sera, three were also positive with p27 M-PMV. The other five were found to be M-PMV negative.