SynopsisIJttrclttted keratins, such as horn and fur, have markedly dissimilar physical properties, and these variations can exist, although to a lesser extent, between two non-identical samples of the same keratin. Each keratin examined contains low-sulfur proteitis which bear resemblance to each other and this is in accord with the concept that all a-keratins contain microfibrils of similar size and structure. However, these keratins can differ greatly in the type and amount of high-sulfur protein which they contain. It is reasonable then to assume that variations in physical properties between keratins must be due, a t least in part, to differences in the proportion or properties of the matrix (high-sulfur protein) which surrounds each microfibril. High-sulfur proteins have been found in every a-keratin examined with horns containing as little as 7% by weight and furs as much as 45% by weight of these proteins. By the supplementary feeding to sheep of cysteine or methionine administered into the abomasum, the high-sulfur protein contetit of wool can be increased by 5075. This opens the possibility of the control of fiber properties by the dietary manipulation of sheep.