We have produced a monoclonal antibody (MAb), designated 11G12, of IgG1 isotype that recognizes group-specific epitopes of the HBsAg. It was demonstrated that the same MAb 11G can be used effectively as both the capture and tracer antibody to detect HBsAg in human plasma samples, using sandwich radioimmune assays (11G-11G sRIA) and sandwich avidin-biotin types of enzymatic assays (11G-11G sABC). The sensitivities for 11G-11G sRIA and 11G-11G sABC is 2.5 ng/ml and 5.0 ng/ml of HBsAg, respectively. Our data suggest that sufficient numbers of the same epitope exist on the HBsAg so that the use of the same MAb as both capture and tracer antibody does not reduce the sensitivity of the assays. The use of MAb 11G as both capture and tracer also minimizes the "hook effect" often encountered in testing high concentrations of plasma HBsAg by other assays. We also experimented with a sandwich RIA in which MAb 11G was used as the capture and polyclonal anti-HBs antibody as the tracer and found that such a combination increased the sensitivity of detection to 1.25 ng/ml of HBsAg.