This study evaluates a new nonangiographic marker of reperfusion--a rapid initial increase in plasma creatine kinase (CK) and CK-MB activity--in 50 patients receiving intracoronary streptokinase. Blood for CK and CK-MB activity was sampled at 30-minute intervals and angiography performed at 15-minute intervals or earlier if there were clinical signs suggestive of reperfusion. An absolute first-hour increase in CK activity of 480 +/- 345 IU/liter (range 54 to 1,440 IU/liter), or a relative first-hour increase of 34 +/- 18% (range 13 to 67% of the peak rise), or an absolute first-hour increase in CK-MB activity of 48 +/- 36 IU/liter (range 10 to 144 IU/liter) or a relative first-hour increase of 27 +/- 13% (range 13 to 57%) was found in patients immediately after reperfusion with Thrombolysis In Myocardial Infarction (TIMI) grade 3 perfusion of the artery of infarction. The onset of rapid increase in CK and CK-MB activity closely reflected the time of angiographic documentation of reperfusion. In contrast, in the absence of reperfusion, the absolute rate of increase in CK activity measured in the last hour of the 2 1/2-hour period beginning with the start of treatment was only 15 +/- 9 IU/liter on the average (range 2 to 30 IU/liter) and the relative rate of rise was 3 +/- 2% on the average (range 1 to 6%).(ABSTRACT TRUNCATED AT 250 WORDS)