['The purpose of this study was to assay reactivity of antibody-positive sera to different parts of the HIV-1 and HIV-2 proteinase (PR) proteins.', 'Since the majority of HIV-1-antibody-positive sera react to the proteinase, but the antigenic determinants on the protein have not been identified, we attempted to identify these determinants.', 'We synthesized 18 peptides representing the PR of HIV-1 and HIV-2 in order to map serum reactivity to the PR protein.', 'Both HIV-1- and HIV-2-antibody-positive sera recognized four distinct antigenic regions in the HIV-1 and HIV-2 PR.'] Are four distinct antigenic regions present in the primary structure of HIV-1 and HIV-2 proteinases?