The incidence of icteric and anicteric hepatitis was determined prospectively in 110 patients undergoing open-heart operations in which cardiopulmonary bypass was used. Patients were supplied with blood from two commercial blood banks (82 patients), or from local volunteer donors (28 patients). Serial determinations of serum transaminase levels were obtained for six months following operation. Hepatitis developed in 51% (42) of the recipients of commercial blood, but the disease did not occur in any patient who received blood from volunteer donors. The hepatitis carrier rate for commercial blood donors was 6.3%, but, for volunteer donors, was less than 0.6%. This difference in carrier rates probably reflects differences in the donor populations.