A set of seven murine monoclonal antibodies were generated against a chemically synthesized 11-kDa 104-mer peptide covering the C-terminal residues 270-373 of the p24 gag protein (HIV-1BRU strain). All monoclonal antibodies recognized HIV-1IIIB infected MOLT3 cells by fluorescence and gave positive Western blot signals with viral gag peptides (p55 and/or p24). Oligopeptide binding regions were located with competitive enzyme-linked immunosorbent assays. Detailed epitope scanning analyses (the Geysen technique) were performed by serological testing of the monoclonal antibodies against 99 overlapping hexapeptides which corresponded to the entire 104-mer region. The antibodies bound to p24 peptide sequences located within the 275-293 and 351-368 regions. One antibody (LH104-B) which reacted with residues 357-362 bound to p55 alone. In contrast, another antibody (LH104-I), which recognized the residues 358-363, i.e. with five out of six residues in common with antibody LH104-B for its epitope region, reacted exclusively with p24. At least two of the antibodies (LH104-C and -A) which bound to p24 alone, apparently recognized conformational epitopes. They gave positive reactions with the regions 288-293/351-356 and 284-289/351-356, respectively. This work shows that chemical synthesis of large peptides is a viable alternative approach to immunochemical studies of viral proteins.