We tested the hypothesis that donor blood containing antibody to hepatitis B core antigen (anti-HBc) but lacking detectable hepatitis B surface antigen (HBsAg) and antibody (anti-HBs)might transmit Type B hepatitis by examining donor and recipient serums from a Veterans Administration study of post-transfusion hepatitis. Donor blood was available from three patients with Type B hepatitis and from one patient with hepatitis B virus infection (development of anti-HBs and anti-HBc) without symptomatic disease. All four had received 1 unit of blood with high titer of anti-HBc but lacking HBsAg and anti-HBs. In contrast, no such units had been transfused into nine patients with "immunization-like" response (development of anti-HBs without anti-HBc) or into 26 control patients. These data stress the importance of anti-HBc as an indicator of hepatitis B virus infection and support the hypothesis that high-titer anti-HBc-positive blood might be infectious.