We report here the results of our evaluation of the effects of a dry heat treatment (96 h at 68 degrees C) to eliminate LAV/HTLV-III virus on factor VIII (FVIII) and von Willebrand factor (vWf) present in an intermediate-purity concentrate. This thermal inactivation appears to have little effect on FVIII. There is an acceptable loss (12.3 +/- 3.6%; n = 25) in FVIII coagulant activity (FVIII: C) and a good in vivo performance in haemophilia A patients. A precise analysis of vWf indicates that whereas the vWf antigen and its ristocetin cofactor activity decrease during heating, there is an increase in potentially functional forms of vWf. Heat treatment induces an increase in high molecular weight forms of vWf and an enhancement in platelet adhesion to collagen. These changes probably explain the correcting effect on the bleeding time of the heated FVIII concentrate in patients with von Willebrand's disease. Thus, this heat-treated concentrate appears to be equivalent to the untreated product in haemophilia A, with the additional benefit of being efficient for the treatment of von Willebrand's disease.