Blood coagulation factors V and VIII each serve cofactor functions with different vitamin K-dependent serine proteases of the coagulation cascade. Physical, physiologic, and kinetic data suggest analogous structures and functions for these two proteins. Proteolytically activated factor V (factor Va) is required for the efficient production of thrombin from prothrombin by factor Xa. Similarly, activated factor VIII (factor VIIIa) performs its cofactor activity with factor IXa to produce the activated form of factor X (factor Xa). The studies reported here on the sequences from the thrombin-activated and unactivated cofactors provide evidence that factor V and factor VIII are chemically related and that the structures of both cofactors involve some tandem duplication.