Modulatory role of whole cardiac myosin binding protein-C (s{cyrillic}MyBP-C) in regulation of cardiac muscle contractility was studied in the in vitro motility assay with rabbit cardiac myosin as a motor protein. The effects of cMyBP-C on the interaction of cardiac myosin with regulated thin filament were tested in both in vitro motility and ATPase assays. We demonstrate that the addition of cMyBP-C increases calcium regulated Mg-ATPase activity of cardiac myosin at submaximal calcium. The Hill coefficient for '. pCa-velocity' relation in the in vitro motility assay decreased and the calcium sensitivity increased when s{cyrillic}MyBP-C was added. Results of our experiments testifies in favor of the hypothesis that s{cyrillic}MyBP-C slows down cross-bridge kinetics when binding to actin.