MYOSIN from rabbit skeletal muscle contains low molecular weight proteins (light chains) as well as the two large polypeptide chains which comprise the bulk of the molecule. Alkali treatment has been used^1–4 to separate the light from the heavy chains, and they have been characterized by gel electrophoresis and in the ultracentrifuge. The molecular weights are still in dispute, being reported as 20,000 daltons² and 32,000 daltons³, but the light chains give at least three bands on gel electrophoresis and these molecular weight determinations^2,3 were made with the mixture.