We established an enzyme immunoassay (EIA) for atrial myosin light chain 1 (ALC1) using monoclonal antibodies KA1 and KB1, which were specific for ALC1 and for both ALC1 and ventricular myosin light chain 1, respectively. The serum ALC1 levels of healthy subjects were 0.28 +/- 0.14 ng/ml (mean +/- SD). The tissue ALC1 levels of normal adult human atria were much higher than those of ventricles (p < 0.01, 2,120 +/- 1,200 in right atria, 2,180 +/- 1,450 in left atria vs. 36.0 +/- 20.2 in right ventricles, 37.7 +/- 15.3 in left ventricles, ng/mg of proteins). The tissue ALC1 levels of ventricular aneurysms were significantly higher than those of normal ventricles (p < 0.01, 206.7 +/- 101.8). These results indicate that ALC1 is augmented in aneurysms and that the EIA provides a useful tool to investigate the roles of ALC1.