The efficiency of heat treatment procedures of factor VIII and factor IX concentrates, prepared from voluntary, non-paid donors by three French Blood Transfusion Centres, on the inactivation of HIV and non-A, non-B hepatitis (NANB) viruses was assessed. Some 43 patients (26 haemophilia A, 17 haemophilia B) were followed for at least 1 year by testing for HIV antibodies and alanine aminotransferase (ALT). No HIV seroconversion was observed indicating that heat treatment was completely efficient. Among 26 haemophiliacs, 6 (4 haemophilia A, 2 haemophilia B) presented an elevation in ALT, indicating only a 75% reduction of NANB viral contamination.