The pattern of the crosslinks in the microstructures of hair was investigated by analyzing the reaction of hair with aqueous KCN by means of chemical and physical methods. It was found that the disulfide (SS) bonds in hair were selectively converted to monosulfide (S) crosslinks by the treatment with 0.08M aqueous KCN solution. The KCN-treated hairs swollen with an 11M LiBr solution containing N-ethylmaleimide showed rubberlike elasticity in a solution composed of equal volumes of 8M LiBr and diethylene glycol monobutyl ether. Stress-strain relations of the swollen hairs were analyzed by applying a rubber elasticity theory. It was demonstrated that : the conversion reactions of SS to S links occur between the low-sulfur (LS) proteins at the initial step of the reaction; SS bond scission between the high-sulfur (HS) proteins commences at a faster rate through the conversion reactions of intermolecular SS bonds to intramolecular S bonds; and the SS bonds within the matrix proteins are less reactive than the intermolecular bonds in the LS proteins. The number and the type of crosslinks in microstructures of the intact hair were also determined. The percentage ratios of the different crosslinks in LS proteins were 27.0% intermolecular SS, 39.0% intermolecular X, and 34.0% intramolecular SS + X links, where X are the crosslinks other than SS links; the values in the HS proteins were 11.9% intermolecular SS and 88.1% intramolecular SS links. The percentages of the number of crosslinks in LS and HS proteins were 13.8 and 86.2% of the total number of crosslinks of hair, 627 mu mol/g, respectively.