The exclusion of commercial and hepatitis B surface antigen (HBSAg)-positive donors, as measured by counterelectrophoresis, has markedly reduced the frequency of posttransfusion hepatitis (PTH). A further, significant reduction in type-B PTH can be achieved by prescreening donors for HBSAg by solid-phase radioimmunoassay (RIA). When a voluntary donor population, pretested by RIA, is used, approximately 90 per cent of residual hepatitis is serologically unrelated to either the type-A or type-B hepatitis viruses. Similarly, cytomegalovirus and the Epstein-Barr virus are not serologically implicated in “non-A, non-8” hepatitis. Additional human hepatitis virus(es) may exist.