Coronary arteriography and biplane ventriculography were performed in 51 patients during the acute (mean of 6.6 hours after onset of symptoms) and chronic (1 to 3 months after admission) phase of myocardial infarction. Twenty-four patients were treated in a conventional manner. In 27 patients, reperfusion was achieved with intracoronary streptokinase after 24 ± 20 minutes of infusion. Peak creatine kinase and cumulative creatine kinase release were derived from serial creatine kinase measurements. Ejection fraction and the length of the akinetic or dyskinetic segments were calculated in the chronic phase. The time interval between onset of symptoms and peak creatine kinase was significantly shorter for the streptokinase-treated patients as compared with the conventionally treated patients (13.5 ± 5.3 versus 22.9 ± 7.4 hours, p = 0.0001). Significant linear correlations were obtained for both streptokinase-treated and control patients, relating: 1) peak creatine kinase value to both length of the noncontracting segment and ejection fraction in the chronic phase, and 2) cumulative creatine kinase release to both length of the noncontracting segment and ejection fraction in the chronic phase. Patients treated with streptokinase experienced a relatively greater release of enzyme for a given infarct size as compared with those treated in a conventional manner. The difference in enzyme release between the two groups increased as infarct size increased. These observations may be explained by enhanced washout of enzyme from the infarct zone, secondary to reperfusion after intracoronary streptokinase therapy. The variability in creatine kinase release observed in the streptokinase-treated patients with small infarcts suggests that prediction of infarct size for an individual patient from time-activity curves of creatine kinase release should be made with caution.