Fresh myocardium homogenates analyzed by thin-layer isoelectric focusing revealed the presence of two prominent creatine kinase (CK; EC 2.7.3.2) sub-bands, MM0 (pI 7.10) and MM1 (pI 6.88), in approximately equal proportion. While these forms represented together as much as 85% of the cellular MM fraction, they accounted only for viz. 2.2 and 27.7% of the total serum MM activity when measured 8 h before the CK peak in patients with myocardial infarction. Incubation of the isolated MM0 and MM1 with normal human serum demonstrated that the former turned to MM1 within 5 h at 37 degrees C; further changes affecting MM1 gave rise to other sub-bands, MM2 (pI 6.70), MM3 (pI 6.45), and MM4 (pI 6.25). In our patient population, these three forms represented more than 75% of the serum CK-MM activity at the CK peak; hence, soon after the enzyme release, the serum MM isoenzyme mainly consists of degradation products arising from the labile MM0 and MM1. Among the two cellular forms, MM0 was the best related to the total enzyme activities and the most efficient for differentiating the patients with left ventricular failure from the others during the entire survey period (F = 3.8, p less than 0.05). Because its presence in the blood provides evidence for a very recent CK release from the tissues, serum CK-MM0 determinations might be proposed for following the extension of the lesion after a myocardial infarct.