The high levels of covalent disulfide crosslinking in keratins strongly affect (1) structural stability, (2) viscoelasticity, and (3) chemical reactivity. This paper briefly reviews recent work on these subjects, with critical emphasis on methods by which chemical and physical properties can be related to inter-and intra-molecular crosslink density in heterogeneous systems like keratins. Detailed attention is drawn to effects of crosslinking on the hydrolysis of keratin by acids or enzymes. Within the limits of reasonable assumptions, it is possible to account quantitatively for crosslink dependent variations in the hydrolysis rate of different keratins, and also to derive a formula for calculating the absolute intermolecular crosslink density from the amount of keratin dissolved after partial hydrolysis and the number of chain ends appearing in the soluble fraction.