Light-scattering studies indicate that 10-3 [image] pyrophosphate and Salyrgan (Mersalyl) increase the radius of gyration of purified myosin B, but does not change the weight-average Mol. Wt. When a myosin B preparation is stored or diluted, its ultracentrifugal Mol. Wt. is considerably decreased upon the addition of pyrophosphate. It is concluded that myosin B consists of 10% myosin A, 70% main component (intact myosin B), elongating by the pyrophosphate addition and 20% dissociable component(s), dissociating by the pyrophosphate addition, having a high viscosity and being separated by ultracentrifugation.