The wells of the polystyrene microplate strips are coated with mouse monoclonal anti-HBs. Both human serum or plasma and polyclonal anti-HBs labeled with horseradish peroxidase are incubated in these coated wells. HBsAg, if present, will bind to the solid phase antibody and the labeled antibody resulting in the HBsAg being sandwiched between the solid phase antibody and labeled antibody. Excess unbound HBsAg and labeled antibodies are removed by washing. Substrate solution containing hydrogen peroxide and 3,3,5,*-tetramethyl- benzidine(TMB) is then added to each well. The presence of HBsAg is indicated by the presence of a blue color after substrate addition. Reaction is terminated by addition of sulphuric acid. The intensity of the color is measured spectrophotometrically at **0nm and is proportional to the amount of HBsAg present in the specimen.