To determine the clinical utility of human ventricular myosin light chain 1 (HVLC1) in the diagnosis of myocardial necrosis, we established a radioimmunoassay for serum HVLC1 using polyclonal antibodies raised against the purified human protein. HVLC1 levels were measured in sera from 110 control patients and 38 patients immediately after cardiovascular surgery and in serial specimens from 10 patients with uncomplicated myocardial infarctions. The HVLC1 assay was found to be equal in sensitivity for the first 48 hours to the CK-MB for myocardial necrosis after cardiovascular surgery and for myocardial infarction. In virtually all myocardial infarct patients, HVLC1 levels in sera rose within hours of the onset of chest pain. From thereon, two sub-trends were discerned. For many, the HVLC1 levels remain elevated for 9-12 days beyond. In some, the HVLC1 levels returned to normal levels 1-2 days after the initial rise, but became elevated again for the next 5-9 days. In either instance, the diagnosis of myocardial infarction was permitted as late as 10-12 days after the onset of chest pain which is in stark contrast to all conventionally used biochemical markers.