Antibodies in the serum reacting with antigens on the surface of radiolabelled Dane particles distinct from hepatitis B surface and core antigens (HBsAg and HBcAg) were detected, using a double antibody precipitation assay, in 12 out of 15 patients early in the course of acute type B hepatitis and at the time of disappearance of circulating Dane particles. No such antibody activity was found in 15 of the 16 patients with HBsAg-positive chronic active hepatitis, 13 of whom had complete Dane particles in the serum. In a group of 16 asymptomatic HBsAg carriers (without Dane particles in serum) antibody activity was shown in nine. This demonstration of antibodies precipitating Dane particles may be relevant to the clearance of circulating hepatitis B virions and the termination of infection in acute type B hepatitis. Their absence in all but one of the cases of chronic active hepatitis might explain why the virus infection persists in this group of patients.