Values for dityrosine concentration were determined in wool and its morphological components, the cortex and cuticle. Dityrosine was found to be located in the tyrosine-rich proteins of wool, cortex, and cuticle. Within the cuticle it was shown to be present in the intercellular cement and the A-layer. Within the cortex it was shown to occur in the intermicrofibrillar matrix where the tyrosine-rich proteins are located. Dityrosine was not an artifact of acid hydrolysis but a natural part of the wool. An investigation of the in vitro formation of dityrosine in wool fibers showed that the concentration of dityrosine increased in the wool fibers upon oxidation with hydrogen peroxide or by UV irradiation. It is suggested dityrosine may be involved in the yellowing of bleached wool fibers. Fluorescence studies showed that wool fibers had a natural blue fluorescence when excited at 365 nm. Dityrosine was unlikely to be responsible for this fluorescence. Upon oxidation of wool fibers the intensity of fluorescence was enhanced, due to the oxidation of disulfide bonds to cysteic acid.