One way in which coagulation may be initiated is via the tissue factor pathway in which a non-proteolytic enzyme activator, tissue factor, complexes with a plasma protein, factor VII. Factor VII is the zymogen of a serine protease, factor VIIa. While factor VII-tissue factor has coagulant activity, upon conversion to factor VIIa the activity increases about 120-fold. We now show that tissue factor markedly accelerates the conversion of factor VII to factor VIIa. Thus, this essential coagulation cofactor is clearly bifunctional.