Isometric tension development of glycerinated single fibers prepared from normal and PSE porcine muscles upon addition of Mg2+-ATP was estimated. The study was also made on the myosin-irrigated fibers prepared by irrigating porcine myosins into rabbit “ghost” fibers. The isometric tension of the fibers was decreased as the pH of the muscle was decreased. The tension development of normal myosin-irrigated fiber was 60% higher than that of the “ghost” fiber, while that of PSE myosin-irrigated fiber was diminished to zero. From the evidences obtained in the present and in the previous studies, we have tentatively concluded that lack of contractility of PSE muscle is mostly due to the irreversible binding of myosin to actin filaments.