The association between hepatitis B antigen (HB antigen; SH or Australia antigen) and long incubation period or “serum” hepatitis is well established. But the significance of HB antigen in the pathogenesis of the disease and its relationship to the infective agent, remain uncertain. The infectivity of hepatitis B virus is more heat resistant than that of most viruses; its serological activity, detected by immunodiffusion, survives treatment at 56° C for 6 h (ref. 1), but is partially inactivated by heating to 60° C for 10 h (unpublished observations) and completely destroyed at 85° C for 1 h (ref. 2). Serological activity in the immunodiffusion test is also remarkably resistant to digestion with proteolytic enzymes and inactivation by protein denaturants1,2; it is, however, affected by powerful lipid solvents, suggesting that intact lipid in the particle is necessary for serological activity in this test. Finally, the antigenicity of HB antigen, although surviving 98° C for 1 min (refs. 3, 4), is of a low order; antibody is not normally detected by standard techniques after recovery from serum hepatitis5,6, although highly sensitive techniques such as radioimmunoprecipitation may enable it to be detected.