An improved method of electrophoresis under nondenaturing conditions separated three electrophoretically distinct isomyosin triplets when applied to rat fast-twitch muscles displaying a predominance of one of the fast myosin heavy chain HCIIb, HCIId or HCIIa. The three isomyosin triplets, named FM1b-FM3b, FM1d-FM3d, FM1a-FM3a, correspond to the three possible alkali light chain (I.C.) combinations (L.C.??? homodimer, LC??? LC3f heterodimer, and LC3f homodimer) with each fast HC isoform. Different proportions of these various isomyosins suggested specific affinities of light chains LC??? and LC3f for the fast heavy chain isoforms.
