ORGAN-SPECIFIC ENZYMES are used in the diagnosis, prognosis, and quantification of tissue destruction in various disease states. The enzyme investigated by Coplin et al1 in this issue of the ARCHIVES describes the use of cerebrospinal fluid (CSF) creatine kinase–BB isoenzyme activity (CK-BB) as a prognostic indicator after subarachnoid hemorrhage (SAH). Creatine kinase is found in 3 isoenzyme forms: CK-BB, predominantly in the brain; CK-MB, in cardiac muscle; and CK-MM, in skeletal muscle.2 Accurate separation and measurement of these isoenzymes has become important in the diagnosis of myocardial infarction (MB) and in various forms of muscle disease (MM). Creatine kinase–BB is predominantly localized in neurons and astrocytes in brain tissue.3 With any destructive process of the brain, CK-BB leaks into extracellular fluid and then into body fluids. CK-BB is also found in smaller amounts in the prostate, urinary bladder, lungs, uterus, stomach, kidneys, salivary gland, thyroid, and pancreas.4 Separation and measurement of CK isoenzymes can be performed by electrophoresis,5 ion exchange chromatography,6 and radioimmunoassay,7 the latter being the most sensitive for measurement of CK-BB.