In the present study the effect of phosphorylation of skeletal muscle myosin light chains on the interaction between myosin and actin has been investigated. The actomyosin ATPase activities were determined for synthetic actomyosins formed from either phosphorylated or non-phosphorylated myosin and pure actin, with the help of the luciferin-luciferase system. The contractile properties of our preparations were simultaneously studied by the superprecipitation model. Phosphorylated form of myosin had lower actin-activated ATPase activity at particular conditions studied. In agreement with this, superprecipitation of phosphorylated actomyosin was delayed. On the basis of these results one can expect that phosphorylation of myosin light chains modulates contractile properties of intact skeletal muscle.