Two hundred and seventy patients under 71 years of age with myocardial infarction less than 4 hours old, defined by clinical and electrocardiographic criteria, were included in this trial and followed up for 1 year: two groups of 89 and 92 patients were randomised to receive APSAC (30 mg i.v. over 5 minutes) or rt-PA (10 mg bolus + 5000 IU of heparin as a bolus, followed by 90 mg rt-PA over 3 hours) and compared with a control series of 89 consecutive patients treated with streptokinase (1.5 MU in 1 hour). Heparin and aspirin (250 mg/day) were prescribed systematically. A score of efficacy was established from the following 4 parameters: patency of the infarct-related artery on coronary angiography at day 6 +/- 2 (N = 252), dyssynergic score on radiological ventriculography, infarct size on resting Thallium myocardial scintigraphy performed between day 15 and 21 (N = 242) and radionuclide ejection fraction performed at the same time. This score (0-24) was respectively 17.8 +/- 6.4 for rt-PA, 17.7 +/- 6.0 for APSAC and 18.1 +/- 6.0 for streptokinase (NS). The costs of hospital treatment were assessed by including: the cost of thrombolytic therapy (ranging from 1.7% of total cost for streptokinase to 16% for rt-PA), the cost of other treatments and biological investigations (10% of total cost); the cost of followed coronary angiography, in 33% of patients, by an angioplasty (21% of total cost), the cost of hospital stay averaging 17 days (49% of total cost in the rt-PA and APSAC groups and 56% in the streptokinase group NS).(ABSTRACT TRUNCATED AT 250 WORDS)