An ELISA-inhibition assay based on commercially available HIV-1 p24 antigen tests was developed for detecting p24 antibodies. The test is specific and simple. p24 antibody was detectable in all p24 antigen negative Western blot positive sera, but was detectable infrequently in antigen positive sera. Sera from patients with indeterminate HIV-1 reactions (individuals without evidence of HIV-1 infection who nevertheless had p24 antibody) were p24 antigen negative and p24 antibody negative in the ELISA-inhibition reaction.