The authors conducted a series of experiments concerning the condition of hypercoagulability of patients with a history of thrombosis, in view of determining whether or not the risk of thrombosis could be assessed by laboratory tests. To that end they assayed antithrombin iii activity and soluble fibrin complexes in their test subjects. Antithrombin activity was assayed by a chromogenic method; soluble fibrin complexes by the hemoagglutination test. The activity of antithrombon III was significantly reduced in patients with venous thrombosis, not so in arterial thrombosis. Testing for soluble fibrin complexes was invariably unrewarding, probably because the thrombosis episodes under investigation were of too long standing.