The brain isoenzyme of creatine kinase (CK) (EC 2.7.3.2) has been identified in the serum of 21 of 950 patients tested. Samples were received over a period of about 20 months. Analysis was performed by electrophoresis on agarose gel with isoenzyme bands visualized by fluorescence after enzyme coupled reactions producing NADH. Specialized substrate preparations were used to rule out the possibility that observed fluorescence was due to enzymatic activity other than that of CK isoenzymes. Patients who were hypothermic and/or had undergone cardiopulmonary resuscitation accounted for 19 of the 21 patients showing the brain isoenzyme. The inciting mechanism for the release of CK1, into the circulation is not clearly understood.