To determine the frequency of post transfusion hepatitis (PTH), in Papua New Guinea, 143 patients were serially blood tested after blood transfusion. Detection of HBsAg and anti-HBs was performed either by RIA or a haemagglutination method, and SGOT levels were measured. Of patients transfused with HBsAg positive blood, 85% developed HBsAg or anti-HBs within two weeks of transfusion. 18% developed subclinical PTH; no cases of clinical jaundice were noted. Of patients transfused with HBsAg negative blood, 12% developed HBsAg or anti-HBs within 2 weeks of transfusion. The frequency of PTH was not statistically different from that of the previous group. This study demonstrates that clinical Type B post transfusion hepatitis must be unusual in PNG but mild subclinical post transfusion hepatitis does occur in a minority of patients transfused with HBsAg positive or negative blood. It is postulated that the low frequency of PTH is related to the normally high Hepatitis B immune status of Papua New Guineans. Minimal health benefits would be conferred by the introduction of sensitive and expensive widespread testing for HBsAg in all donor blood; scarce financial resources may be better diverted to other areas of more productive health care.