We evaluated the performance of a monoclonal radioimmunoassay (M-RIA) with enhanced sensitivity and high specificity for hepatitis B surface antigen (HBsAg) in blood donors. The results were compared to a conventional RIA that used polyvalent antibodies (P-RIA). Analysis of 6409 American blood donors not reactive by P-RIA revealed an additional 1.4 HBsAg-positive donors per 1000 by M-RIA, or an approximate 60 percent, improvement in HBsAg detection rate. Furthermore, in 995 Israeli blood donors negative by P-RIA, 11 additional HbsAg-positive donors were identified. The 55 percent improvement in detection rate was similar to that observed with American blood donors. Since several of the newly identified HBsAg-positive blood donors had antibodies to the core antigen (anti-HBc) as the only serologic evidence of recent or past hepatitis B exposure, we studied an additional 68 anti-HBc-positive individuals with the M-RIA. It was found that 26 percent (18/68) reacted only by M-RIA and not by P-RIA. These findings suggest that there are blood donors with HBsAg undetectable by P-RIA.