The hepatitis B virus (HBV) is believed widely to be noncytopathic in its natural host, man, although this may not be true in animal models, for example the transgenic mouse where expression of HBsAg is associated with the onset of hepatocellular degeneration and necrosis. The basis for this belief is simply that in the earlier stages of chronic HBV infection, when viral replication is most efficient, 'patients' remains asymptomatic and a liver biopsy reveals no evidence of inflammation or parenchymal liver damage. The major features of immunological interest in HBV infection are first that a proportion of those exposed to the virus become chronically infected and second that once chronic infection is established, most will gradually eliminate the nucleocapsid antigen from the liver, while the envelope antigens continue to be expressed. During the elimination of nucleocapsid antigens, many patients develop permanent liver damage.