The current experiments examine the reaction between purified human alpha-thrombin and purified human factor VIII and compare this with the reaction between alpha-thrombin and the low molecular weight factor VIII procoagulant activity obtained by high ionic strength dissociation. The reaction patterns of both procoagulants are similar, demonstrating in initial increase in factor VIII activity followed by a decay of procoagulant activity. The extent of activation which is observed in the initial phase decreases with lower temperatures and with lower thrombin concentrations. A pseudo-first order relationship is demonstrated for the activation phase. The labile factor VIII procoagulant activity produced by the initial action of thrombin appears to be intrinsically unstable, since the addition of hirudin or diisopropylfluorophosphate does not prevent the subsequent decay of the procoagulant activity. The similar activation and decay patterns of purified factor VIII and the dissociated low molecular weight factor VIII procoagulant support the concept that the low molecular weight procoagulant itself represents the functional coagulant moiety of the factor VIII complex.