Exclusion of acute myocardial infarction preoperatively, particularly in patients undergoing cardiac catheterization, is an important requirement for optimal results following coronary revascularization. Unfortunately, activity of conventionally measured serum enzymes (AST, LDH, total CPK) is frequently raised because of enzyme released from non-cardiac sources during the catheterization procedure. However, serum activity of the MBCP Kisoenzyme, an isoenzyme found primarily in heart muscle, appears to be more specific. Accordingly, in the present study, total CPK and MB CPK activities were determined in serum samples from 53patients undergoing diagnostic catheterization, immediately before study and serially for 24 hours afterwards. A comprehensive range of catheterization procedures included selective coronary arteriography in 39 patients by brachial (I7) or femoral (22) artery approaches. Myocardial infarction was excluded by clinical and electrocardiographic criteria in all patients before and after the procedure. MB CPK isoenzyme activity was also measured in serum samples from 5o patients with acute myocardial infarction documented electrocardiographically, and in 20 controls admitted to hospital but without cardiovascular disease. In patients with acute myocardial infarction, both total CPK and MB CPK isoenzyme levels were significantly raised (o.78 ± o.o87 and o.o86 + 0.037 IU/ml, respectively), exceeding the upper limit of normal in all cases. MB CPK activity remained within normal limits (< o0.4 IU/ml) in all 20 subjects without cardiovascular disease. Peak total serum CPK activity exceeded control levels in all patients undergoing catheterization (0.260± o.o33). However, in each case, MB CPK isoenzyme activity remained within normal limits (<.oo4). Thus, in contrast to an increase of activity of conventionally used serum enzymes, increased MB CPK isoenzyme activity is a reliable indicator of myocardial infarction, even in patients undergoing cardiac catheterization.