We measured Thrombin/Antithrombin III Complex (TAT) in the plasma from normal subjects and the patients with various diseases by a two-step sandwich enzyme immunoassay. The measurements were performed periodically for the patients with acute promyelocytic leukemia with disseminated intravascular coagulation (DIC). During the course of DIC, the levels of TAT varied according to the activity of DIC. TAT level was so sensitive that it could detect latent activation of the clotting pathway. Besides DIC, marked elevation of TAT level was found in some patients with advanced cancer, collagen diseases with vasculitis and infectious diseases. We conclude that the measurement of TAT is valuable for the assessment of activation of clotting pathway.