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J. Soc. Cosmet. Chern. 27 289-300 (1976) ¸ 1976 Society of Cosmetic Chemists of Great Britain The chemistry of human hair cuticle•III: The isolation and amino acid analysis of various subfractions of the cuticle obtained by pronase and trypsin digestion J. A. SWIFT and B. BEWS* Synopsis--The course of digestion of human HAIR CUTICLE by PRONASE and TRYPSIN has been followed both gravimetrically and by the ELECTRON MICROSCOPE examination of digested hair sections. This has permitted the identification of three subfractions of the hair endocuticle. The significance of the AMINO ACID analyses is discussed. INTRODUCTION In the first two papers of this series we described a new technique for the physical isolation of cuticle from human hair (1) and the enzymatic isolation of the A-layer and cell membrane complex from the cuticle (2). Our attention has now turned to an examination of those cuticular components dissolved by the proteo~ lytic enzymes trypsin and pronase. Although Bradbury and Ley (3) have shown already that pronase can be used for the isolation of whole endocuticle from wool, the present experiments are more comprehensive and have enabled us to * Unilever Research Isleworth Laboratory, 455 London Road, Isleworth, Middlesex. 289