Hepatitis B antigen (HB Ag) in the sera of 135 blood donors and patients with either acute viral hepatitis or chronic liver disease was subtyped to better characterize the epidemiology of type B infection. Antigenemic volunteer blood donors had HB Ag which was predominantly subtype D (D:Y ratio of 7:1) whereas this subtype was found in only slight excess (D:Y ratio of 1.8:1) in the sera of patients acquiring hepatitis after transfusion of blood drawn entirely from the volunteer donor population. Patients with "needle stick hepatitis" had almost exclusively subtype Y-HB Ag. In contrast, individuals with chronic liver disease had primarily subtype D-HB Ag. Patients without recognized exposure ("sporadic hepatitis") had a HB Ag subtype D:Y ratio of 1.2:1. Using demographic data, it was possible to distinguish subgroups related to the antigen subtype. The age, race, sex, and socioeconomic status of patients with sporadic hepatitis and subtype D-HB Ag were similar to individuals with post-transfusion hepatitis, whereas patients with subtype Y-HB Ag had epidemiological features similar to persons with needle stick hepatitis and frequently gave a history of close contact with individuals using drugs illicitly.