Seven monoclonal antibodies were obtained after immunization of mice with purified sodium dodecyl sulfate (SDS)-disrupted feline immunodeficiency virus (FIV). Six antibodies specifically bound antigens in the cytoplasm of FIV-infected cells as determined by indirect immunofluorescence and reacted with FIV p24 gag gene product in immunoblots. One reacted positively with virus-infected cells, but failed to recognize FIV structural proteins by immunoblotting. Using competition binding studies, the anti-p24 monoclonals were shown to detect four distinct B-cell epitopes. Competition with sera of FIV-infected cats showed that such epitopes are immunogenic also in the natural host species.