A monoclonal antibody, MAb H24, recognized a Plasmodium falciparum antigen with a relative molecular mass (Mr) of 52 kD that appeared to be a rhoptry component by immunofluorescence microscopy. The antigen is synthesized during both ring and schizont stages, but pulse-chase experiments showed that it is not carried through to the next ring stage after reinvasion. It was not labeled by 3H-glucosamine. The purified MAb failed to inhibit parasite invasion in vitro. The antigen was isolated using affinity chromatography, and used to produce a monospecific polyclonal antibody (PAb H24) in mice. Polyclonal antibody H24 recognized the same antigen as MAb H24 as judged by both immunofluorescence microscopy and immunoprecipitation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and was markedly inhibitory in vitro.