We report the case of a patient with persistently above-normal activity of creatine kinase (CK) in serum, a major fraction of which on electrophoresis moved as a band between the MM and MB isoenzymes and on anion-exchange column chromatography eluted in the MB fraction. Measurements in the presence of specific M or B subunit-inhibitory antibodies indicated that 93% of the activity consisted of B-isomers. From these experiments we conclude that the abnormal CK is of BB nature. Gel filtration and immunoglobulin precipitation showed that the CK-BB was complexed with IgG. Normal CK-BB, when mixed with the patient's serum, was converted to macro CK-BB. In vitro stability of 37 degrees C of the abnormal enzyme was much greater than that of normal BB and MM isoenzymes. Following this finding, we then assessed 310 sera, received for enzyme assay by the clinical laboratory, for electrophoretically abnormally migrating CK isoenzymes. Of these, five (1.6%) contained such enzymes, all being of BB nature. They were of increased molecular mass, and at least three of them were complexed with IgG.