Muscle contraction results from cyclic attachment and detachment between myosin head in the thick filament and actin in the thin filament. According to biochemical studies on the actomyosin ATPase reaction, the myosin head (M), in the form of M●ADP●Pi, first attaches to actin (A), and exerts a power stroke, associated with release of ADP and Pi, so that at the end of power stroke, M forms rigor linkage AM with A. Upon binding with a new ATP, M detaches from A to extert a recovery stroke, associated with the reaction, M●ATP → M●ADP●Pi, and reattaches to actin. The presence of the rigor linkage AM during muscle contraction is, however, not yet proved by X-ray diffraction studies on contracting muscle fibers. To give information about formation of the rigor linkages, we examined development of rigor linkages in single skinned rabbit psoas muscle fibers in response to removal of ATP from the relaxing solution by recording changes in muscle fiber stiffness, measured by applying sinusoidal vibrations (peak-to-peak amplitude, 0.2% of L0; 2kHz) at 20oC. If 4mM MgCl2 was present in the ATP-free rigor solution, the fibers exhibited a long latent period of 40-60s until starting to develop appreciable stiffness, which then increased slowly to a peak with a half rise time of 20-30s. By the addition of three different monoclonal antibodies (against catalytic, converter and lever arm domains, respectively; Sugi et al. BBRC 405: 651-656, 2011), both the latent period and the half rise time of rigor stiffness development increased by a factor of 1.5-2. When Mg ions in the ATP-free rigor solution was removed by 10mM EDTA, The latend period for rigor stiffness development markedly decreased to 2-3s, and the antibodies had no effect on the latent period, but still increased the half rise time of rigor stiffness development by a factor of 1.5-2. The result that the rigor linkage formation is retarded by the antibodies seem to imply that, in the present experimental conditions, the rigor linkage formation requires some structural changes in the catalytic, converter and lever arm domains, so that binding of antibodies to these domains retards the rigor linkage formation. Meanwhile the antibodies had no effect on the relaxation of rigor force and stiffness when rigor fibers were returned to the relaxing solution containing 4mM ATP.