Isoforms of creatine kinase (creatine-N-phosphotransferase, CK, EC 2.7.3.2), BB and MB, were isolated from healthy human brain tissue and cardiac muscle, respectively, and were characterized by means of isoelectric focusing (IEF). CK-BB isoforms in Tris-HCl buffer were focused at pI 4.5 (a tissue form) and those in fresh sera from healthy adults were focused at pI 5.0, 5.1 and 5.2 (plasma forms). The IEF patterns of CK-BB isoforms were not altered following treatment with carboxypeptidase B and incubation in fresh serum at 37 degrees C; thus, it was found that there was no lysine at the C-terminal of the CK-B subunit and CK-BB isoforms were not results of the removal of lysine. Three CK-BB isoforms in fresh sera were identified to be oxidized, intermediate and reduced forms from the anodal side, respectively, by treatment with hydrogen peroxide and 2-mercaptoethanol. The oxidized form of CK-BB seemed to have higher affinity to IgG than the other two plasma forms of CK-BB. On the other hand, CK-MB isoforms in Tris-HCl buffer were focused at pI 5.4 (a tissue form) with a minor band at pI 5.2 and those in sera were focused at pI 5.0, 5.1, 5.2 (plasma forms) and 5.4. Four CK-MB isoforms were identified to be reduced, intermediate and oxidized forms without lysine from the anodal side, respectively, and the cathodal band was a tissue form with lysine.