The aim of this study was to determine the dose-response relationship of anisoylated plasminogen streptokinase activator complex (APSAC) by means of a between group double-blind comparison of the new agent and placebo. 50 patients with symptoms of acute myocardial infarction of less than 6 hours duration and whose coronary artery occlusion had been confirmed by coronary angiography were randomly allocated to 5 treatment groups (15, 20, 25 or 30U of APSAC, or placebo) and treatment was given as an intravenous injection over 2 minutes. Angiography was performed again at 15, 30, 45, 60 and 90 minutes and the films were assessed centrally by 2 independent cardiologists. Six patients were excluded from the angiographic analysis, 4 because their angiograms revealed patent arteries before APSAC was administered, 1 patient was excluded because streptokinase had been infused just after randomisation because of cardiogenic shock and 1 because the angiogram was not available due to problems in the film development. Clinical and laboratory examinations carried out for 72 hours after treatment showed that the drug was well tolerated. The reperfusion rates were as follows: placebo, 0/9; APSAC 15U, 5/8; APSAC 20U, 5/9; APSAC 25U, 6/9; APSAC 30U, 5/9. Reperfusion was achieved in 60% of treated patients but no dose relationship was revealed.