ABSTRACTKeratin and prekeratin were extracted from human plantar epidermis and carried on SDS‐PAGE. Prekeratin in the basal as well as spinous cell layer of the epidermis was composed of 6–7 bands (M.W.: about 71,000‐49,500), while keratin in the surface of stratum corneum was composed of 3–4 bands (M.W.: about 66,500–53,000), indicating molecular modulation of prekeratin during epidermal cell differentiation.Amino acid compositions of prekeratin and keratin were similar, glycine, glutamic acid and serine being major three amino acids. Since the keratin molecule is known to be composed of a repeating amino acid sequence [Fuchs&Green:Cell,15, 887–893 (1978)], our result indicates that the modulation of molecular weight of prekeratin during differentiation might be due to enzymatic splitting of peptides of these