Studies of the coagulation and fibrinolytic enzyme system in menopausal women on estrogen replacement therapy revealed a statistically significant increase in concentrations of fibrinogen and α1-trypsin inhibitor. A decrease in plasminogen was noted, using a fibrinolytic assay and antithrombin III activity (assayed by a coagulation technic). There was no change in Factor VII, X or plasminogen by radial immunodiffusion, α1-macroglobulin, plasmin inhibitor, platelet number and platelet adhesion. Fibrinolysis breakdown products and fibrin monomers were not detected. The present study supports the assumption that estrogen is the effective hormone to initiate changes in the coagulation and fibrinolytic enzyme systems in women taking oral contraceptives.