Abstract By the Archibald approach to sedmentation equilibrium method, we have found a molecular weight of 4.2·10 5 g for either, (I) myosin (A) or, (2) the principal, ( ca . 65% by weight), schlieren-visible component of 5-h extracted myosin B. This same value has also been estimated from the conventional sedimentation velocity-diffusion method, using sedimentation coefficients obtained at very low protein concentrations, and diffusion coefficients measured by observing boundary spreading in synthetic boundary cells. Although lower than hitherto reported molecular weights for myosin (A), this value is in line with minimum molecular weight estimates based on the present knowledge of myosin sub-units.