Since the myosin light chain kinase (MLCK) is found in association with actin filaments in skeletal muscle, the author investigated the regulatory role of the actin-binding activity of MLCK from chicken breast muscle in the actin-myosin interaction. The amount of MLCK bound to actin increased with an increase in the concentration of MLCK to the 10-7M level. The actin-activated ATPase activity of myosin in both the phosphorylated and the unphosphorylated form was reduced to the same extent with an increase in the concentration of MLCK. Since MLCK showed little binding to myosin, the inhibition of the ATPase activity was attributed to the actin-binding activity of MLCK and not to its phosphorylating activity. The actin-binding activity of MLCK was obscured when Ca2+ and calmodulin (Ca2+-CaM) were present, and the inhibition by MLCK of the ATPase activity was also abolished. The inhibition of the ATPase activity and the recovery from the inhibition upon addition of Ca2+-CaM were confirmed by a in vitro motility assay in which ATP-dependent movement between actin and myosin can be detected directly. The author has discussed the domain that is responsible for the inhibitory effect.