All of 6 heat-treated prothrombin complex concentrates (PCC) tested contained adequate levels of factor IX but factor VII content was low. Levels of factors II, X, protein C and protein S were variable and antigen levels were always greater than those of functional activities. On crossed-immunoelectrophoresis factor IX showed variable anodal shift in all concentrates tested and in some activated factor IX was demonstrated by immunoblotting technique. These findings suggested some activation and/or denaturation during production and/or heating. Modest amount of factor VIII clotting activity by solid-phase amidolytic method and of factor VIII antigen was demonstrated in some concentrates but none contained more than a trace factor VIII inhibitor bypassing activity. The results suggested that heat-treated PCC should provide safe therapeutic products for hemophilia B.