Abstract. Studies were undertaken to enhance the sensitivity of a previously developed RPHA test for HBsAg. A net increase in sensitivity of approximately 3‐fold was achieved by modifying the elution procedure used to purify chimpanzee anti‐HBs by affinity chromato‐graphy. A further 3‐ or 4‐fold sensitivity increase was achieved by increasing the volume of specimen tested. A concomitant increase in nonspecific agglutination usually observed with increased specimen size was avoided by incubating the reaction mixture at 37 or 45 °C. Evaluation of the test in detection of HBsAg in blood obtained from volunteer donors indicates that the materials produced at the New York Blood Center using the modified test protocol compare favorably with a commercial RPHA test. Modifications which did not contribute toward enhancing sensitivity are also reported.