Antigenicity in mice of a recombinant polypeptide including the complete amino acid sequence of mature human immunodeficiency virus type 1 p24 protein was studied by induction of monoclonal antibodies (MAbs). A panel of nine recloned hybridomas secreting MAbs with anti-p24 reactivity was isolated and further characterized. Competitive inhibition experiments suggested that the MAbs could be grouped into four epitopic classes corresponding to at least two distinct determinants. Analysis of reactivity to recombinant p24 deletion variants indicated that all the recognized epitopes are localized within a carboxy-terminal domain (amino acids 168-208) which should be largely exposed in recombinant as well as authentic antigen. Lack of response to N-terminal and central portions of p24 suggests that the antigenicity of those regions in the natural polypeptide is strongly conformation-dependent.