In a multicentre randomized open study conducted on two parallel groups the effectiveness of APSAC was compared with that of streptokinase (SK) in 116 cases of myocardial infarction treated during the first 2.75 hours. APSAC (30 IU) was administered by intravenous bolus injection over 2 to 5 minutes, and SK (1.5 million IU) by intravenous infusion over 60 minutes. The patency of the coronary artery responsible for myocardial infarction was evaluated by coronary arteriography performed 1.74 h on average after the beginning of treatment; it was 70 p. 100 in the APSAC group and 51 p. 100 in the SK group (p less than 0.05). The fall in plasma fibrinogen was similar in both groups (mean minimum level; 0.2 g/l). Haemorrhages occurred in 9/58 patients treated with APSAC (15.5 p. 100) and in 13/58 patients treated with SK (22.4 p. 100); these haemorrhages took place during the first 24 hours in 4 patients of the APSAC group and in 10 patients of the SK group. Five patients died: 2 in the APSAC group and 3 in the SK group. In a subgroup of 38 patients who underwent 3 control coronary arteriographies (at 90 min, 24 hours and 3 weeks), the patency rates were 63 p. 100, 82 p. 100 and 93 p. 100 respectively with APSAC and 44 p. 100, 86 p. 100 and 92 p. 100 respectively with SK (NS). No coronary reocclusion occurred in the APSAC group, as against 3 (1 early, 2 delayed) in the SK group. It is concluded that APSAC seems to be more effective than intravenous streptokinase; it is easier to administer (bolus injection) and does not carry a higher risk of haemorrhage.