We measured creatine kinase (CK) isoforms by a new immunoinhibition method to evaluate their usefulness in detecting early coronary reperfusion. Blood samples were collected at 15-minute intervals from 50 patients with acute myocardial infarction. CK isoforms were determined by a 10-minute immunoinhibition method with an autoanalyzer. Values for inhibited isoforms (MM3, MM2/2, and MB2/2) were divided by those of noninhibited isoforms (MM1, MM2/2, MB1, MB2/2, and BB) to calculate the isoform ratio. In the reperfused group the increase in the isoform ratio was 2.69 +/- 1.80 (SD) 30 minutes after reperfusion and 2.41 +/- 2.01 at 60 minutes, which was significantly higher than the corresponding values in the nonreperfused group (0.17 +/- 0.16 and 0.32 +/- 0.26, respectively). When an increase of 0.70 or more in the isoform ratio was used as the criterion for reperfusion, the sensitivity and specificity were 92% and 100% at 30 minutes and 100% and 100% at 60 minutes after recanalization, respectively. We conclude that the isoform ratio obtained by the new 10-minute assay of CK isoforms is useful for the noninvasive detection of reperfusion 30 and 60 minutes after recanalization in acute myocardial infarction.