Aliquots from units of blood were screened by radioimmunoassay (RIA) and reverse passive hemagglutination (RPHA) shortly after transfusion. Recipients of positive units, as well as control subjects, were followed prospectively for evidence of either post-transfusion hepatitis (PTH) or exposure to hepatitis B (HB). There was a significant correlation between receipt of either an RPHA-positive or RIA-confirmed positive unit and the subsequent development of PTH or HB exposure. Recipients of all volunteer units had a lower incidence of PTH than did recipients of all commercial units. The frequency of PTH and HB exposure was not statistically different in groups receiving 1 to 5, 6 to 10, 11 to 20 and greater or less than 21 units of blood. The PTH seen in the study presented over a wide clinical spectrum, but it was usually anicteric and frequently asymptomatic.