During a prospective study in post transfusion hepatitis 222 blood units were given to 55 patients who underwent open heart surgery. Eleven of these blood units contained anti-HBs titered higher than 1:128 by passive hemagglutination and were given to 10 patients previously negative for anti-HBs. Eight of these 10 patients showed anti-HBs in the first serum sample obtained usually one to three days after surgery and a continuous decline in titre was noted at later bleedings until antibodies were no longer detectable 2 to 20 weeks after transfusion. In four patients from whom regular blood samples were available the half life of passively acquired anti-HBs was determined to be between 23 and 28 days. In two patients with serum anti-HBs before blood transfusion no change in antibody titre was noted over a period of six months.