The value of different serological assays of hepatitis B virus associated antigens and antibodies was tested in forty-eight consecutive cases of acute hepatitis negative for HBsAg by agargel diffusion. The tests included radioimmunoassay of HBsAg, IgM and IgG anti-HBs and counter-immunoelectrophoresis for anti-HBc detection. Twelve cases were identified as hepatitis B (25%). Eleven cases were positive for HBsAg by RIA, and only one case was detected by an increasing titre of antibodies alone. Further seventeen patients had an unchanged antibody titre to one or more antigens. It is concluded that a sensitive test for HBsAg is the most important tool for diagnosing acute hepatitis B infection.