CK-MB isoenzyme fractionation has been compared between DEAE-sephadex chromatography and the CK-M-subunit inhibiting antibody procedure. 2. It was found that the inhibiting antibody method generally gave lower values than the chromatography procedure. 3. The above difference can be partially attributed to inhibitory factors in the antibody system. 4. It was concluded that although there was general agreement between the two procedures, the CK-M-subunit inhibiting antibody procedure should be viewed with caution at low creatine kinase activities, and should never be done without the appropriate reagent blanks.