The isoenzymes of creatine kinase have been measured in serum and selected tissues from horses. The distribution followed that reported in other species in that the MM dimer of the isoenzyme was present in voluntary and non-voluntary muscle, thyroid, liver, spleen, lung and intestine. The BB dimer of the isoenzyme was predominant in brain, pancreas, kidney, intestine, lung, spleen, liver and thyroid. In contrast, in 4 hearts examined less than 1.5 per cent of the total creatine kinase activity was attributable to the MB form of the isoenzyme. The MB isoenzyme was, however, present in intestine and spleen. In this series the isoenzymes were separated by ion exchange chromatography and their presence confirmed by cellulose acetate electrophoresis. It was concluded that as insignificant quantities of MB isoenzyme are present in equine heart, measurement of this isoenzyme in serum cannot be used to detect myocardial or cardiac damage in the horse. Further detailed studies need to be undertaken to assess the clinical relevance of the isoenzyme pattern in serum or other body fluids, if the measurement of creatine kinase isoenzymes in serum is to be used as a diagnostic aid.