In 1978, we (Suzuki et al., J. Biochem. 84, 1529) reported the very interesting finding that in a medium of 0.2 M KCl (and 10 mM MgCl2), addition of ATP induced a large increase in the sedimentation velocity of chicken gizzard myosin from approximately 6S to 10S. Moreover, our flow birefringence study suggested that 10S-myosin was not much different from 6S-myosin in the particle length. Therefore, we concluded that ATP induced dimerization of gizzard myosin monomers. In the present study, we reinvestigated 6S-myosin and 10S-myosin by the sedimentation equilibrium method, and found that both myosins had the same molecular weight of approximately 500,000. We also studied the angular dependence of the light scattering intensity, and found that addition of ATP caused a large change in the radius of gyration of gizzard myosin; the radius of gyration of 6S-myosin was 545 A whereas that of 10S-myosin was only 146 A. Accordingly, our previous conclusion had to be withdrawn. Instead, we now put forward a new conclusion that ATP induces a large change in the conformation of gizzard myosin monomers. We added two new observations: (a) The ATP-induced change in the myosin conformation and the large decrease in the ATPase activity of myosin were both reversible upon increasing the KCl concentration from 0.2 M to 0.3 M. (b) The large decrease in the ATPase activity and the ATP-induced increase in the sedimentation velocity were also observed with gizzard heavy meromyosin when the KCl concentration decreased to lower than 0.3 M.