To determine whether myocardial infarction could be detected early after onset by analysis of subforms of the MM isoenzyme (isoforms) of creatine kinase (MM CK) in plasma, we subjected eight conscious dogs to coronary occlusion and quantified isoforms in serial plasma samples by chromatofocusing. The fractions of MM(A) (isoelectric point [pI] = 7.91), MM(B) (pI = 7.74), and MM(C) (pI = 7.51) in plasma samples before coronary occlusion averaged 11.4 ± 4.8% (SD), 22.3 ± 5.5%, and 66.3 ± 9.6% of total MM CK activity. The fraction of MM(A), the isoform of MM CK found in myocardium, increased significantly in plasma 1 hr after coronary occlusion, reached a maximum of 49.7 ± 8.0% in 4.1 ± 1.3 hr, and returned to baseline in 12.0 ± 2.3 hr. The fraction of plasma MM CK activity attributable to MM(C), an isoform formed slowly in plasma from MM(A) via MM(B) as an intermediate, decreased significantly within 1 hr, reached a minimum of 14.0 ± 4.1% in 4.8 ± 1.1 hr, and returned to baseline in 13.0 ± 2.9 hr after coronary occlusion. Total CK activity did not increase significantly until later, i.e., 5 hr after occlusion, and peaked at 1371 ± 530 IU/liter in 10.9 ± 1.9 hr. Within the first 4 hr after coronary occlusion, MM(A) consistently comprised more than 20% of plasma MM CK activity despite insignificant increase of total CK. Changes in isoform proportions were consistent and independent of peak total CK activity and of cumulative CK release over a 10-fold range. Thus initial CK release indicative of infarction is detectable within 1 hr after the onset of ischemia by quantification of plasma MM CK isoforms.