Thrombin and natural antithrombin antibody content was comparatively studied in donors and myocardial infarction patients. It was shown that the level of thrombin and antithrombin antibody was stable in the group of clinically normal subjects. In patients with myocardial infarction an increased thrombin level attended by a decrease in the level of natural antithrombin antibody was recorded. The presence of linear correlation between the levels of thrombin and antithrombin antibody, as well as impossibility of the diagnosis of disorders in the blood coagulation system using one of the parameters studied have permitted introduction of an additional diagnostic parameter--"antithrombin index."