Human brain-type (BB) creatine phosphokinase (CPK) and skeletal muscle (MM) CPK were purified from autopsy tissues. The stability of each isozyme with and without the reducing agent mercaptoethanol in human plasma, saline solution, and saline solution with bovine serum albumin was studied. Human BB CPK, at 37 C, without added mercaptoethanol, lost 50% of its activity in 15 minutes during incubation in plasma. It was much more stable at both room temperature and 4 C. BB CPK was also unstable in saline solution plus bovine serum albumin in the absence of mercaptoethanol but was much more stable in the presence of mercaptoethanol. Plasma factors with molecular weights less than 1,000 were potent inhibitors of CPK activity. Electrophoresis following incubation of BB CPK in plasma without mercaptoethanol revealed a new band with CPK activity with the electrophoretic mobility close to that of cardiac muscle (MB) CPK, but no band with the electrophoretic mobility of MM CPK was noted. A new band whose electrophoretic mobility was intermediate between that of MM CPK and that of MB CPK appeared after 21 hours of incubation of MM CPK in human plasma. Modifications of methods for handling blood samples in order to increase the possibility of preserving CPK isozymes in human plasma are proposed. Immunologic methods for determining CPK isozymes may provide more definitive answers as to the nature of the isozyme species present in human sera in various diseases.