The discovery of Australia antigen and subsequent identification as the surface antigen of Hepatitis B virus (HBsAg) represents a significant breakthrough in the understanding of the disease. Screening of blood donors for the presence of this antigen in serum has significantly reduced the incidence of Hepatitis B in blood transfusion recipients. Furthermore, anti-HBsAg determination can be used to: a) monitor the progress of patients recovering from Hepatitis B viral infection; b) indicate if the patient has had prior immunological exposure to HBsAg; and c) study the epidemiological factors associated with transmission of HBsAg.