111 patients below 70 years old, with the first acute myocardial infarctions, 6 hours since the pain occurred--have been treated with streptokinase i.v. In 102 patients we obtained full curve of CK-MB activity. Early peak of CK-MB activity < 15 hours after onset of symptoms we have observed in 59 patients, and late peak of CK-MB activity > 15 hours in 43 patients. There was not any significant statistics differences between early and late groups in frequency of: early ventricular fibrillation (< 48 hours), complex ventricular arrhythmia (in 21 day), heart failure and in-hospital mortality. 1 patient died in hospital in early group and in late group also died 1 patient. The follow-up period was from 10 to 48 months (av. 26 +/- 13). 100 patients left the hospital and the full informations we have obtained in 97 cases. No one died in that time. In the group with early peak CK-MB activity we observed more often the unstable angina and the new myocardial infarction (21%) than in the group with late peak of CK-MB activity (15%), but these differences were nonsignificant. In conclusion our results don't confirm that the early peak of CK-MB activity is the positive risk factor of unstable angina and the new myocardial infarction.