A study was made of the effect of a single intravenous injection of streptodecase and heparin in the treatment of elderly and senile patients with myocardial infarction during short-term (under 6 h) and late (over 30 h) periods of development. It was confirmed that streptodecase had a specific selective effect on the fibrinolytic system of the blood, with the risk of the development of hemorrhagic and thromboembolic complications being minimal. Allergic reactions to streptodecase were recorded in 1 out of 12 patients examined. It was marked that streptodecase produced an insignificant prolonged effect while the drug action lasted maximally 48 to 72 h since the moment of administration. The effect of streptodecase depended on the degree of atherosclerotic involvement of the vessels and was more remarkable in elderly and senile persons. No significant differences were found in the efficacy and duration of the action of the drug administered during short-term and late periods of myocardial infarction development. It is not excluded that streptodecase can be used in higher doses or repeatedly in the immediate 36 to 48 h in the treatment of certain patients groups. The combined use of streptodecase and heparin was discovered to be mandatory for restricting the effect of secondary hypercoagulation.