In recent years evidence has emerged that most post-transfusion hepatitis is caused by one or more previously unknown agents named non-A, non-B. A prospective investigation was made of 74 patients who underwent open-heart surgery. Only volunteer blood was used for transfusions. Transfusion-associated hepatitis appeared in 15 (20%) of the patients 4-12 weeks after the operation. In no case was the hepatitis found to be caused by hepatitis B, A or Epstein-Barr virus. One patient had a cytomegalovirus infection; the other 14 cases (19%) were classified by definition as non-A, non-B hepatitis. Although most of the patients were asymptomatic and all were anicteric, the course of the hepatitis was protracted in many cases. Thus, 6/12 observed patients still had pathologic transferase values more than a year after the onset of hepatitis. Liver biopsy was performed in 3 cases and showed histologic signs of chronic active hepatitis in all of them.