The changes in the mechanical properties accompanying the reduction of disulfide (SS) linkages in hair were studied. A variety of extents of thiol groups were introduced into hair by treatments with thioglycolic acid and tri-n-butyl phosphine. The reduced fibers swollen with an aqueous 11M LiBr solution containing N-ethylmaleimide showed typical rubberlike elasticity in a solution composed of equal volumes of 8M LiBr and diethylene glycol mono-n-butyl ether. The crosslink density was determined from the shear modulus of the swollen fiber. It was found that the SS linkages can be divided into two groups : the intermolecular linkages group (SS1 and SS2) and the intramolecular linkages group (SS3). The effect of the SS bond scission on the shear modulus of the reduced fibers in water was also studied. It was further found that intermolecular SS1 linkages do not respond to the shear modulus of fiber in water, and the amounts of SS1, SS2, and SS3 are about 35, 18, and 47% of the total cystine content in hair (623 mu mol/g), respectively. With respect to the location of SS linkages in hair, important suggestions were obtained : the intermolecular SS1 and SS2 linkages are located in the microfibril and the matrix, and the former are more accessible to water than the latter; and SS3 linkages are localized within the hydrophobic region of the matrix.