A solid-phase radioimmunoassay (Ausria-125) was compared to an immunoelectro-osmophoresis method in detecting hepatitis-B antigen (HBAg). The radioimmunoassay detected HBAg in approximately 230 times higher dilutions than the immunoelectroosmophoresis method. In cases of acute hepatitis HBAg could be detected for about 70% longer after the onset of jaundice using radioimmunoassay compared with immunoelectroosmophoresis. Of 138 patients with acute hepatitis, who were HBAg-negative by immunoelectroosmophoresis, 23 (17%) were positive by radioimmunoassay. The specificity of the positive results with this test was investigated by neutralization tests with human antiserum to HBAg.