Background: Little is known about the relationship between myocardial infarction and the coagulation inhibitors protein C, protein S, and antithrombin III (ATIII). Methods: We explored the hypothesis. that partial deficiencies in protein C, protein S, and ATIII may be associated with acute myocardial infarction by comparing the baseline levels of these anti coagulants in a sample of participants in the Thrombolysis in Myocardial Infarction Phase II Trial (TIMI II) to the levels in a sample of Red Cross donors. We also examined the changes in concentrations of hemostatic and inhibitory factors during thrombolytic therapy, the correlations between the inhibitors and plasminogen and fibrinogen, and the association of baseline inhibitor levels with the degree of plasmin generation during treatment. Levels of protein C, free and total protein S, ATIII, fibrinogen, tissue-type plasminogen activator, (t-PA), plasminogen, and alpha-2 antiplasmin were measured in plasma samples obtained at baseline before treatment with thrombolytic therapy and at 50 minutes, 5 hours, and 8 hours after the start of therapy (n=92). Protein C, free and total protein S, and ATIII were also measured in plasma samples from comparable Red Cross donors (n=92). Results: Baseline protein C and ATIII levels were higher in the TIMI II subjects than in controls (3.7 µ/ml vs 3.3 for protein C, P=0.002; 118% vs 111% for ATIII, P=0.006). Free and total protein S levels were lower in the TIMI II subjects than in controls (4.3 µ/ml vs 5.2 for free protein S, and 15.3 µ/ml vs 17.2 for total protein S, P=0.001). There were no major changes in protein C or protein S levels after initiation of thrombolytic therapy, but there was a 13% decrease in ATIII. Significant correlations were found between baseline protein C, Protein S, ATIII, and plasminogen; none of these factors were correlated with baseline fibrinogen concentration. The pretreatment levels of the inhibitors were correlated with markers of the amount of plasmin generated during therapy. Conclusions: It is not likely that partial protein C or ATIII deficiency is commonly associated with myocardial infarction. It is more likely that at the time of infarction, these inhibitors are somewhat higher possibly due to a homeostatic response to previous plaque rupture and thrombosis. Partial protein S deficiency may play a role in coronary thrombosis. The levels of these inhibitors at the time of infarction may be important in the efficacy of thrombolytic therapy.