The products arising from cystine residues in wool on uv irradiation are cysteine, cysteic acid, and probably an unidentified intermediate oxidation product which reacts with mercurial reagents, also sulfate and “bromide-oxidizable sulfur.” By controlling the conditions of yellowing an attempt has been made as far as possible to separate the chemical effects associated with yellowing from those of general photochemical degradation. Yellowing of wool can be effected with only very slight decomposition of cystine residues (and also some decomposition of tyrosine, tryptophane, and histidine residues). There is no obvious relationship between the extent of cystine decomposition and the degree of yellowing produced.