Creatine phosphokinase (ATP: creatine N-phosphotransferase, EC 2.7.3.2) is the major constituent of the "low-salt-soluble" proteins of the electric organ from Torpedo marmorata. The denatured subunits of the enzyme have an apparent Mr of 43 000 and isoelectric points ranging between pH 6.2 and pH 6.5. Identical properties are found for the creatine phosphokinase from Torpedo muscle tissue. Anti-(electric organ creatine phosphokinase) antibodies are specific for the muscle-type enzyme and do not cross-react with enzymes present in Torpedo brain and electric lobe tissue. Biochemical and immunochemical properties of the enzyme associated with acetylcholine-receptor-enriched membranes show that this enzyme is as the "low-salt-soluble" electric organ enzyme of the muscle-specific type. In vitro translation of electric organ poly(A)-rich mRNA in a reticulocyte lysate reveals the abundance of mRNA specific for muscle creatine phosphokinase. During embryonic development of the electrocyte a continuous increase of translatable amounts of this mRNA is observed. No brain-type polypeptides are synthesized. The subunits of the brain-specific enzyme differ in molecular mass (Mr approximately equal to 42000) and isoelectric properties (pI approximately equal to 7.0-7.2). The unexpected finding that the brain forms are more basic than the muscle-specific enzyme is supported by agarose and cellulose acetate electrophoresis and ion-exchange chromatography properties.