The in vitro properties of 5 heat-treated Factor VIII (F VIII) concentrates were studied and compared with those of conventional non heat-treated concentrates with special reference to F VIII/von Willebrand Factor (vWF) activities. The heat-treated F VIII concentrates were found to contain about 25 U/ml of F VIII: C as indicated on the label, 59.5-103.3 U/ml of F VIII: Ag, 20.0-74.0 U/ml of ristocetin cofactor activity (RCof), and 67.3-96.0 U/ml of vWF: Ag. The F VIII/vWF activities in the heat-treated concentrates were almost the same as those in the 6 non heat-treated concentrates. The antihemophilic factor (AHF)-cryoprecipitate contained 2.3 U/ml of F VIII: C which was roughly equivalent to the amount of F VIII: Ag. In addition, it contained 7.3 U/ml of RCof which was equivalent to the amount of vWF: Ag. The ratio of F VIII: Ag to F VIII: C in the F VIII concentrates ranged from 2.5 to 4.4. The ratio of vWF: Ag to RCof in the concentrates ranged from 1.1 to 4.8. There were, however, no significant differences in F VIII/vWF activities between the heat-treated and the non heat-treated F VIII concentrates with two exceptions. These findings suggest that the inactivation of the biological activities of F VIII and RCof, and/or the denaturation of both the F VIII: C protein and the vWF protein antigen occurred during the production procedure, while the heat-treatment did not lead to further changes of F VIII/vWF.(ABSTRACT TRUNCATED AT 250 WORDS)