Plasma cardiac myosin light chain I(CMLCI) was quantified by competitive enzyme-linked immunosorbent assay(ELISA) using a monoclonal antibody in 42 patients with confirmed acute myocardial infarction (AMI). In comparison to the value from healthy individuals, plasma CMLCI levels were significantly elevated in 40 patients. In 28 of the 42 patients (66.7%), two major peaks were observed on time activity curves. The early peak (CMLCIp1) began a rapid rising within 4-12 hours and peaked at a mean of 25 hours following the onset of chest pain. The CMLCIp1 appeared statistically higher and earlier in patients with early infarct-related artery recanalization (IRAR) than those without IRAR, which was assessed by CPK peaking time, continuous ST segment monitoring on 12 lead ECG and symptoms. Thus, CMLCIp1 might be influenced by early IRAR. The late peak (CMLCIp2), which was composed of another gradual increase of plasma CMLCI level, occurred at a mean of 142 hours after AMI and remained elevated for about 7-10 days. The magnitudes of CMLCIp2 were correlated with the infarct size estimated by LVEF and LVWMS on cineventriculography and QRS scores on ECG. In the rest 14 patients only one peak was observed. It was suggested that CMLCIp1 could provide an early diagnosis of AMI and might be influenced by early IRAR after thrombolysis, while CMLCIp2 was a good later marker of extent of myocardial necrosis.