Factors affecting fibrin formation and dissolution were compared for 15 women taking combined oral contraceptives and 15 women using nonpharmacological methods of birth control. The two groups were matched for age, body weight, time of blood collection, and day in menstrual cycle; none of the women was receiving other drugs known to affect the blood coagulation or fibrinolytic parameters measured in this study. Fibrinogen concentrations tended to be higher in the experimental group; the degree of fibrinogen degradation, number of fibrin cross-links, and levels of factor XIII and plasminogen were the same for both group. There were significant reductions in antithrombin activity, the euglobulin lysis time, and fibrinolytic inhibitor level in women using oral contraceptives. An estrogen dose effect was suggested for fibrinogen concentration and the degree of antithrombin activity. The increased fibrinolytic activity and decreased fibrinolytic inhibitor levels are consistent with in vitro observations that antithrombin also inhibits plasmin activity. Thus while oral contraceptive-induced depression of antithrombin III could possibly predispose to thrombosis by diminishing the inhibition of the serine protease clotting factors, the concomitant decreased level of plasmin inhibition might balance the system by favoring thrombolysis as well as the digestion and inactivation of certain clotting factors by plasmin.