After a first exposure to factor VIII concentrates, 9/9 British patients treated with U.S.A.-derived commercial products, and 10/12 treated with British volunteer (NHS) products, developed acute non-A, non-B (NANB) hepatitis. Hepatitis following commercial products was more severe, and of shorter incubation. High previous exposure to NHS blood products seemed to prevent NHS but not commercial factor VIII-induced hepatitis; the latter was also not attenuated by administration of U.S.A.-derived commercial immune serum globulin (ISG). After a first exposure to NHS factor IX concentrates without ISG, 4/4 patients developed short incubation NANB hepatitis; one also contracted prolonged incubation hepatitis B. One patient treated with ISG and factor IX of proven infectivity did not develop hepatitis, suggesting protection by ISG. Observed differences between concentrates might be attributable to their content of different NANB agents, but dose-related effects could provide alternative explanations. This data provides a basis for comparative assessment of new products of possible reduced infectivity in only small numbers of patients.