To understand the cross-linked structure of hair and wool keratin fibers, rubber elasticity theory was used to model the force-extension curve of swollen fiber in a diluent mixture of concentrated aqueous lithium bromide solution and pure diethylene glycol mono-n-butyl ether. A two-phase structural model was assumed for the globular matrix keratin-associated protein (KAP) dispersed in the swollen network of intermediate filament (IF) proteins. Structural parameters were obtained by fitting force-extension curves to theoretically derived relations between elastic forces originating from nonuniform network and extension ratios. The parameters extracted are the number of intermolecular SS bonds in IF, the volume fraction of matrix proteins in the fiber, and the shape of the matrix domain. The number, type, and location of SS cross-linkages on an IF protein chain were estimated using the results obtained in the reactivity of SS bonds of wool and hair fibers in boiling water and also in the behavior of SS bonds in permanent waving treatment of hair. Boiling water data were mainly based on the reactivity of SS bonds in self-cross-linking reactions forming stable lanthionine and lysinoalanine cross-links, and the data for the permanent wave hair were based on the scission mechanism in reduction with thioglycolic acid and subsequent reformation of the SS bonds during oxidation. A near-perfect reformation of the SS bonds in the IF molecules were attained in the latter, but the reformability of the bonds between the KAP molecules was very low. To assess the cross-linked structure of KAP component proteins, permanent wave hairs were used and the results analyzed were presented for the verification of this model. The total amount of SS cross-links is twenty-one moles on an IF chain of average molecular mass of 50000. Out of the twenty-one moles, thirteen moles are intermolecular cross-linkages and eight moles are intramolecular cross-linkages. A KAP molecule with an assumed molecular mass of 20000 involves seventeen moles of intramolecular SS bonds and four to five sites on the surface of the KAP molecule of hair keratin. These sites link to adjacent KAP molecules and form an aggregate of about six KAP molecules against an IF molecule in the hai