A monoclonal enzyme immunoassay for measuring human ventricular myosin light chain isotype 1 (HVMLC1) in serum has been developed. To evaluate the method in patients with suspected myocardial injury, we studied 51 patients (16 acute myocardial infarction (AMI), 19 unstable angina pectoris (UAP), 9 stable angina pectoris, 3 nonischemic heart disease, 4 hip surgery patients), and 190 controls (blood donors). Serial blood-samples were drawn from patients; a single blood-sample from controls. The diagnostic value of the HVMLC1 assay was compared with total creatine kinase (CK), CKMB activity, CKMB mass concentration, lactate dehydrogenase isoenzyme 1 (LD1), troponin T (TnT) and mitochondrial-aspartate aminotransferase (m-ASAT). The detection limit of HVMLC1 was 0.4 microgram/l (linear range 0-20 micrograms/l). Sera from 190 reference persons did not contain detectable levels of HVMLC1 (< 0.4 microgram/l; 99% percentile). The coefficients of variation were 13% (1.0 microgram/l) and 3.1% (17.7 micrograms/l). Cross-reactivity with myosin from skeletal muscle was seen. Times to peak value were: CK 19.3 +/- 2.0, LD1 43.4 +/- 3.2, HVMLC1 72.9 +/- 7.0, and m-ASAT 67.3 +/- 5.6 h. Time-curves of HVMLC1 and m-ASAT were similar, whereas time-curves for HVMLC1 and TnT were quite different in most cases. Peak value of HVMLC1 was five times higher than CK peak value and eight times that of LD1. HVMLC1 appeared in the blood within hours after the onset of chest pain and in the majority remained for more than a week after AMI. Among patients with UAP 16% (3/19) had elevated HVMLC1 in serum, whereas elevated TnT was seen in 26% (5/19) and elevated CKMB mass in 26% (5/19). We conclude that the new HVMLC1 assay offers a sensitive diagnosis of myocardial injury. It is characterized by a wide diagnostic time window. The similarity of the HVMLC1 and m-ASAT curves indicates that it may be used to estimate the extent of myocardial necrosis.