Using purified recombinant human ventricular myosin light chain 1 (HVMLC1) as the antigen,clonal antibodies could react with the ventricular myosin light chain 1 isolated from different sources, such as human, rat or pig. It was also demonstrated that C8 was directed against the NN part of the mino acid 99-195).The affinity constants of C8, C9 and B12 were 3.20×108, 8.60×107 and 1.77×108 M-1, respectively,ive ELISA. The isotype of B 12 was determined as IgG2a, whereas the isotype of both C8 and C9 were IgG1. In the presence of C9 or B12, the actin-activated Mg2+ ATPase activity of myosin ase activity for C8. B 12 and C9 also inhibited the superprecipitation of porcine cardiac native actomyosin (myosin B) and reconstituted actomyosin,ate that all three monoclonal antibodies could bind the intact myosin molecule,but B 12 and C9 might more easily react with epitopes located in the C-fragment of HVMLC1. The inhibitory light chain 1, particularly the C-fragment domain, is involved in the modulation of the actin-activated Mg2+ ATPase activity of myosin and,as a consequence,plays an essential role in the interaction of actin and myosin.