The dynamic fluctuation of thrombin-antithrombin III complex (TAT) was studied in blood obtained during the daytime (at 9 AM, noon, 3 PM), during extracorporeal circulation and during the course of disseminated intravascular coagulation (DIC), to certify whether the level of TAT in blood can reflect the generation of thrombin. In 10 healthy male volunteers, the mean values of TAT (micrograms/liter) were 1.74 (+/- 1.36) at 9 AM, 1.22 (+/- 0.47) at noon, and 1.25 (+/- 0.68) at 3 PM. TAT did not show a daytime fluctuation, unlike fibrinolytic factors. The mean values of TAT in 38 hemodialyzed patients were 4.83 (+/- 2.8) before the initiation, 6.59 (+/- 4.39) in the first hour, and 13.42 (+/- 10.96) at the end of a dialysis session. In 20 patients undergoing open heart surgery, the mean value of TAT was increased during cardiopulmonary bypass (CPB) and decreased with time after the end of surgery. The fibrinopeptide A (FPA) value was increased with TAT during CPB but achieved a maximum level immediately after heparin neutralization by protamine. In 20 patients with DIC, the values of TAT varied from 5.8 to 297 micrograms/liter in the blood at the onset of DIC. In seven of eight patients treated with low-molecular-weight heparin (LMW-H), the values of TAT and FPA were lower 24 hr after LMW-H than before the treatment. These results suggest that the level of TAT in blood reflected the formation of thrombin and could serve as a sensitive parameter of activated coagulation in circulating blood.