Radioimmunoprecipitation of Dane particle core radiolabeled in a DNA polymerase reaction was used for detecting anti-HB(c) in the sera of blood donors positive for HB(s)Ag and patients with acute hepatitis B. The test for anti-HB(c) was a specific and sensitive indicator of current or recent virus infection. Human anti-HB(s) hyperimmune immunoglobulin contained small amount of anti-HB(c), but the antibody was not detected in the serum of hyperimmunized chimpanzees.