Monoclonal antibodies (MAbs) were raised against gag proteins of human immunodeficiency virus type 1 (HIV-1), strain HTLV-IIIB. One of 29 antibodies was specific for p17 of HIV-1. Twenty of 28 MAbs reactive with the major core protein p24 of HIV-1 showed cross-reactivity with HIV-2, and five of these also detected the corresponding antigens of simian immunodeficiency virus (SIVmac). The MAbs were reactive in several tests, i.e. ELISA, immunostaining of Western blots, immunofluorescence, alkaline phosphatase-anti-alkaline phosphatase immunocytochemistry and immunoelectron microscopy. The submembrane protein p17 was clearly localized within the virion.