Isoelectric focusing of serum creatine kinase (CK;EC 2.7.3.2) reveals up to 14 CK-MM subbands following acute myocardial infarction (AMI). The "normal" subbands 1 (pI 6.91), 2 (pI 6.65) and 3 (pI 6.35) are faintly present in normal serum and the "abnormal" subbands c (pI 7.25), e (pI 6.85), g (pI 6.50), i (pI 6.28), j (pI 6.20) and k (pI 6.15) are prominently detected in sera with elevated CK. "Abnormal" subbands a (pI 7.55),b(pI7.35),d(pI7.05),f(pI6.72) and h(pI6.40) have only been detected in AMI. The "abnormal" subbands appear, and reach maximum intensity (together with CK-MM 1-3), 3-12 h after infarction, and become faint and anodally convert (as do CK-MM 1-3) within 36 h. Similar changes are detected by nonequilibrium pH gradient electrophoresis which combines CK-MM and CK-MB analysis. In vitro incubation of serum with 0.015 M 2-mercaptoethanol induces conversion of CK-MM 1, 2 and 3 to b and c, d and e, and f and g, respectively. Thus, the complexity of the patterns is explained by a secondary conversion of "normal" to "abnormal" subbands superimposed upon anodal conversion of CK-MM 1----3. The clinical significance of these findings is discussed.