The efficacy and safety of a monoclonal purified factor VIII concentrate (Hemofil M) were assessed in a historically controlled study in 22 HIV-negative patients with haemophilia A, previously treated with various concentrates. Data from 5 years of follow-up were compared with those from the preceding 6 months. Factor VIII consumption remained unchanged. A temporary increase in the number of reported bleedings was attributed to more frequent follow-up visits in the first year. Allergic reactions, inhibitor formation and HIV infection were not seen. Liver function parameters fluctuated in individual patients, and were not related to the ultrapure concentrate used. No clinical evidence of liver insufficiency was seen. The number of CD4-positive lymphocytes was stable, while platelet numbers showed a remarkable increase. We conclude that in previously treated HIV-negative haemophiliacs, treatment with a monoclonal purified factor VIII concentrate is efficacious and safe with regard to HIV transmission, allergic reactions, induction of inhibitors, and deterioration of immune abnormalities.