B-Cell function was evaluated in a group of 43 patients with factor VIII or factor IX deficiency. Thirty had been treated primarily with cryoprecipitate and 13 with concentrates of factor VIII or IX. Serum immunoglobulin G levels were found to be diffusely elevated; however, the absolute number of mature B cells in peripheral blood was normal. B-Cell function as measured by testing mitogen-induced proliferation and and immunoglobulin secretion by plaque-forming cells (PFC) in vitro was reduced. Coculture experiments suggested that these abnormal B-cell responses might be secondary to increased suppressor T-cell activity, which was found more frequently in patients seropositive for antibody to lymphadenopathy-associated virus. Both seronegative and seropositive patients had reduced responses in the proliferative and PFC assays, but the lowest PFC responses occurred in the seropositive group.