We could detect skeletal muscle injury early after an acute exercise bout by measuring creatine kinase (CK, EC 2.7.3.2) MM isoforms in serum. Eleven men performed 120 alternating-arm, eccentric (muscle lengthening) biceps contractions with the intensity of each contraction being 110% of maximal concentric strength--a form of exercise previously shown to cause significant increases of CK in serum at 24 h and muscle soreness 48 h after exercise. Total CK and CK-MM isoform activities in serum were determined before and at 0.5, 0.75, 1, 1.5, 2, and 6 h after exercise. Using thin-film agarose gels and a rapid isoelectric focusing technique, we separated the MM isoforms into MM3 (skeletal muscle form), MM2, and MM1 (in vivo conversion forms). The isoforms reflected the MM form released into the serum from tissue as well as the conversion of one form to another. There were no significant increases in total CK from before to 6 h after exercise: 75 (SD 36) vs 91 (SD 33) U/L. However, CK MM3 in serum increased significantly (P less than 0.01) within 2 h after exercise from 22 (SD 6)% to 28 (SD 6)%. The MM3 to MM1 ratio also increased significantly (P less than 0.05) during this time, from 0.6 (SD 0.3) to 0.9 (SD 0.4). Thus, quantification of CK MM isoforms permitted very early detection of skeletal muscle enzyme release.