Proteins from biopsy of human heart muscle (n = 250) were studied. The supplementary fraction was found in a material from an individuum that coincided in molecular mass but differed in pI from the light chain of myosin usually expressed in the ventricular tissue of the heart muscle (LCM-1 v). The two-dimensional electrophoresis and immunoblotting have shown this supplementary fraction to be a rare allele of LCM-1 v.