Modifications of Factor VIII related antigen (F. VIII R. A.) were investigated in three hemophilic patients during the course of post transfusion hepatitis. F. VIII R. A. was found to be elevated and correlated with the increase of the S.G.P.T. During the acute phase of the disease the Factor VIII related protein was tested for its ristocetin cofactor activity, its electrophoretic mobility on crossed antigen‐antibody electrophoresis and its elution pattern on Sepharose 4B columns; all these properties proved to be abnormal.Liver function tests and F. VIII R. A. were measured in 36 multitransfused hemophiliacs. There was a significant increase of the F. VIII R. A. in the plasma of patients with abnormal liver function tests. Only a few of the multitransfused hemophiliacs had clinical symptoms of hepatitis, although 72 % had elevated transaminase level