Abstract: An assay which is responsive to the levels of anti-p24 antibodies in serum samples is provided which comprises the steps of: (a) forming a test mixture comprising the sample and an antigen solution containing free p24 antigen within a predetermined concentration range: (b) incubating the test mixture under conditions whereby anti-p24 antibodies from the sample, if any, can react with the free p24 antigen to form antibody-antigen complexes; (c) determining the concentration of free p24 antigen remaining in the test mixture after the incubation; (d) determining the concentration of free p24 antigen in the antigen solution; and (e) calculating the difference between the concentration of free p24 antigen in the antigen solution and the concentration of freee p24 antigen in the test mixture after the incubation.The difference calculated in step (e) represents the "binding capacity" of the serum sample for p24 antigen.