Post-transfusion hepatitis developed in 2% of 842 cardiac-surgery patients surveyed in Sydney (4 cases per 1000 units of transfused blood). 3 of the 18 cases were caused by hepatitis B virus even though all units of blood which contained hepatitis B surface antigen (HBsAg) had been rejected. 1 case was caused by cytomegalovirus, and there were 14 (78%) cases of non-A, non-B hepatitis. A significantly higher proportion of the units of blood given to the patients in whom non-A, non-B hepatitis developed contained antibodies against both hepatitis B core antigen and HBsAg than the units of blood given to the other patients. Rejection of blood with these markers of past exposure to hepatitis B may reduce the incidence of post-transfusion non-A, non-B hepatitis by up to a half.