We have investigated 8 cases of clinically manifest hepatitis type B said to be posttransfusion hepatitis that have become known in the years 1978-1981. Paired samples of serum from all 54 blood donors implicated have been drawn at the time of the donation in question as well as a few months later. Only 1 donor, negative for HBsAg but positive with high titer for anti-HBc, could have been a possible source of infection. There is strong evidence that this person is a chronic low level carrier. In the case of 2 other patients transmission of hepatitis by clotting factors from pooled plasma cannot be denied. Regarding the remaining 5 cases, there is no hint whatsoever for a causal connection between blood transfusion and hapatitis type B.