In this controlled multicentre trial treatment with either streptokinase or heparin was allocated at random to patients suffering from myocardial infarction of less than 24 hours' duration. Treatment with either drug was standardized and lasted for 24 hours. A total of 764 patients entered the trial; 34 patient charts were rejected (including all 28 charts from one centre) because of data failure. On retrospective analysis of the 730 remaining patients the two groups were found to have been comparable at the start. The total hospital mortality was 18·5% of 373 patients allotted to streptokinase treatment and 26·3% of 357 given herapin. The mortality after infusion (24 hours) was 10·6% of 340 patients treated with streptokinase and 17·8% of 320 given herapin (P=0·011). Reinfarction in hospital after the 24-hour period of infusion occurred significantly less often in patients treated with streptokinase (P=0·036). Bleeding from puncture sites and pyrexia occurred more frequently during streptokinase treatment. After exclusion of those patients whose diagnosis was unconfirmed on retrospective assessment, the total hospital mortality rate was 19·0% of 357 patients treated with streptokinase and 27·4% of 339 treated with heparin (P=0·011). These results indicate that in recent myocardial infarction streptokinase was superior to heparin in reducing mortality and reinfarction rate during an average period of six weeks in hospital.