Staying in the tradition of Astbury's hypothesis about the role of the α ⇔ β transformation for the stress/ strain-curve of wool fibers and of Feughelman's X/Y-zones model, the interrelation between the morphological structure of keratin fibers and the shape of their stress/strain-curve in water is reevaluated. The yield and post-yield regions can be attributed to the opening up of two distinctly different and well defined portions of the monomer of the intermediate filament; the increased slope in the postyield region can be attributed to the influence of the sulfur bonds in one of the segments.The role of the sulfhydryl-disulfide interchange reaction for the appearance of the post-yield region is pointed out and the molecular mechanisms for achieving the maximum possible strain are discussed.J. B. Speakman was the first prominent protagonist of the theory that the mechanical properties of wool fibers have an important influence on their processing performance. In consequence, he placed a major emphasis on measuring stress/strain-curves and their changes with physical and chemical modifications [ 29 ] . The upper curve in Figure 1 shows somewhat schematically the stress/strain curve of a wool fiber in water. FIGURE 1. Stress/strain curve of a wool fiber in water (schematically ) and the components of the two-phase model. Stress and strain are not drawn to scale. _ On the molecular level, structural investigations using x-ray diffraction led to the discovery of axially oriented a-helices by Pauling and Corey and of the ahelix b 0-pleated sheet transformation with strain by Astbury and Woods. Studies by transmission electron microscopy revealed the existence of lightly stained cylindrical structures, traditionally referred to as &dquo;microfibrils,&dquo; embedded in a more darkly stained &dquo;matrix&dquo; phase [ 16 ] .On the basis of knowledge of the microscopic and molecular morphology of a-keratin fibers, major attempts have been made, especially by Hearie et al. [ 21,22 ] and Feughelman [ 15 ] , to consistently interpret the shape of the stress/strain-curve in relation to fiber structure. Both these approaches are based on the twophase model proposed by Feughelman [ 10 ] , where the filament phase dominates the stress/strain curve in the wet state, while the amorphous matrix phase either plays a minor role [ 15 or exhibits a significant contribution only at high strains [ 21,22 ] .Si nce these versions of the structure / property relationships [ 15,22 ] , more detailed information has become available on the structure of the &dquo;microfibrils,&dquo; in modern semantics referred to as &dquo;intermediate filaments.&dquo; This has enabled us, when expanding a previous account of our research [ 39 ]. to reevaluate the models on a molecular level. The model refinement, which stays in the tradition of Feughelman's X/Yzones model [ 1 l ,15 ] , achieves an improved consistency of the property / structure relationship. Structural Principles and Stress/Strain Properties of Fibrous Keratins a-Keratin f...