This study is designed to: 1. Establish the value of Hepatitis B antigen in detection of potentially infectious blood donors and to define the true usefulness of screening for this factor in relation to the total problem of post-transfusion hepatitis. 2. Develop and evaluate optimal methods of testing for Hepatitis B antigen. 3. Discover other agents which may be involved in the production of post-transfusion hepatitis. 4. Investigate the role of Hepatitis B antigen in the pathogenesis of acute chronic hepatic damage. 5. Evaluate the possible interactions of this antigen with immune factors in the pathogenesis of hepatitis and other diseases.