Because paradoxical increase in thrombin activity was reported after the administration of streptokinase in patients with acute myocardial infarction the velocity of reperfusion and degree of myocardial damage were studied when heparin was infused during rather than after streptokinase infusion. Thirty seven consecutive patients with acute myocardial infarction were randomised to receive intravenous heparin during (group 1, n = 18) or after (group 2, n = 19) streptokinase (1.5 megaunits over 60 minutes). Markers of reperfusion were monitored every 15 minutes for 3 hours. The serum concentration of creatine kinase was measured every 2 hours. The two groups were similar in terms of age and sex distribution, infarct site, time to treatment, and baseline myocardial ischaemia. Patients in group 1 had a significantly shorter mean (SD) reperfusion time (57 (35) minutes v 101 (47)). From 60 to 120 minutes after randomisation there were significant differences in ST segment elevation between the groups. Serum creatine kinase MB peaked earlier (8 (2) hours) in group 1 than in group 2 (10 (4) hours). The peak concentration was significantly lower in group 1 (87 (47) mU/ml) than in group 2 (134 (96) mU/ml) and infarcts were smaller (25.2 (9.8) gram equivalents/m2) in group 1 than in group 2 (35.1 (10.2) gram equivalents/m2). Simultaneous infusion of heparin and streptokinase speeds up the appearance of signs of reperfusion and reduces infarct size.