Plasma protein C (PC) levels were measured in 15 controls and 30 patients with myocardial infarction (Ml). In 19 of them an additional examination of PC level was performed on the 10th day of Ml. There was no significant difference between control and the patient group in the first assay. However PC levels were found to be higher than control in the patient group on the 10th day. These results confirm a significant involvement of blood clotting system in ischemic heart disease but exact mechanism remains