Studies were undertaken to characterize the nature of hepatitis B antigens and their respective function in hepatitis B virus infection. Hepatitis B surface antigen (HB#Ag) was isolated from the plasma of asymptomatic chronic carriers by several methods including centrifugation, affinity chromatography and isoelectric focusing. Analysis of purified antigen revealed the presence of both lipid and protein together with significant levels of carbohydrate. A heterogeneity was demonstrated for HB Ag and this was found to be related in part to the organisation of the protein moiety of the antigen. Hepatitis B core antigen (HBcAg) was isolated from the plasma of a proportion of carrier plasma. Activation of an associated DNA polymerase active on an endogenous template permitted the radiolabelling of HBcAg. Owing to the close association of the reaction product, such preparations were found suitable for use in a radioissnune procedure for the detection of antibody to this antigen. The possible location of all or part of the viral genome is discussed in viral antigens and their expression during the relation to type B virus antigens and their expression during the course of infection.