Deep-vein thrombosis (D.V.T.) was detected by the fibrinogen-uptake test in six out of a total of thirty-one young women undergoing emergency abdominal surgery who gave a history of recent oral contraceptive intake. In contrast, no D.V.T. developed in nineteen similar patients who were not on oral contraceptives (P less than 0-01). Plasma-antithrombin-III activity was significantly lower preoperatively in patients taking oral contraceptives; postoperative D.V.T. subsequently developed in three out of five patients with preoperative antithrombin-III activity below 50%. In seventy-eight dental patients undergoing molar extraction, antithrombin-III activity was measured before, during, and after operation. Activity fell in all patients during operation, but the fall was significantly greater in women taking oral contraceptives (P less than 0-01). The intra-operative fall in antithrombin-III activity was prevented by a small preoperative dose of subcutaneous heparin.