We describe an atypical form of creatine kinase in the serum of a woman after myocardial infarction. Electrophoresis on agarose gel showed a single fraction between CK-MM and CK-MB isoenzymes. After ion-exchange chromatography on DEAE-Sephadex A-50 we isolated the atypical CK and CK-BB as detected by electrophoresis of the eluted fractions. Results of our immunological investigations and exclusion chromatography clearly demonstrate that the atypical CK consists of complexes with high molecular masses formed by CK-BB isoenzyme and immunoglobulin G. The clinical significance, if any, of this Macro CK has yet to be determined.