Two pregnant women with hereditary antithrombin deficiency were treated with subcutaneous heparin for prolonged periods and with a concentrate of purified antithrombin during periods of thrombogenic stress (labour, delivery, abortion). Blood samples obtained at brief intervals during thrombogenic stress revealed concomitant fluctuations in plasma antithrombin levels, coagulation times, and platelet counts. The observations show that the effects of heparin and antithrombin are more complex than hitherto assumed.