We present analytical and clinical studies of three commercial immunoassays for the determination of the creatine kinase MB isoenzyme (CK-MB). All methods are compared with the presently-accepted reference method, agarose gel electrophoresis. We find that each of the immunoassay methods offers diagnostic sensitivity and specificity similar to those of electrophoresis, while none experiences detectable interference from high concentrations of the creatine kinase BB isoenzyme (CK-BB) or the so-called "atypical" bands. The presence of atypical bands can cause electrophoresis to yield inaccurate quantification or even misidentification of CK-MB. Because the immunoassay approaches are more rapid than electrophoresis and require less technical expertise, it is possible to evaluate patients in a more timely and cost-effective manner. Further, since immunoassay results can be available in approximately 1/2 hour, rapid real-time decisions can be made regarding the desirability of initiating or continuing such valuable but dangerous procedures as thrombolytic therapy.