The influence of streptokinase (SK) on myocardial infarct size and left ventricular function after acute myocardial infarction was investigated. 21 patients with myocardial infarction received SK (SK-group), 27 patients underwent conventional therapy (C-group). In both groups therapy started within 8 hours after onset of chest pain. In the SK-group initially 250 000 IU were administered intravenously, followed by a maintenance dose of 100 000 IU/h, lasting 15 hours. Blood samples at 8 hours intervals were collected for 3 days for serial CPK-analysis to calculate infarct size (I=∫f(t)×dt×K×bw). M-mode echocardiography was taken before start of t her a py and after 15, 24, 48 and 72 hours. AOP and heart rate were recorded continuously. Infarct size was 47±12g in the SK-group and 84±25g in the C-group (p<0.05). The average time to peak blood CPK-activity was 24 hours in the SK-group and 40 hours in the C-group. Peak CPK-level was significantly higher (p<0.5) in the SK-group (841±160U/l) than in the C-group (532±13 8 U / l ) . In 16 patients of the SK-group short periods of ventricular tachycardia were recorded during the period of fibrinolysis. Before therapy all patients showed abnormal motion of the posterior left ventricular wall and/or the interventricular septum, detected by echocardiography. 14 patients showed after fibrinolysis an improved or normalized motion.The results indicate that early fibrinolysis may reopen the occluded coronary artery. Reperfusion of the ischemic perfusion area may salvage jeo pardized myocardium, therefore infarct size was reduced and ventricular function improved.