The temperature dependence of the mechanical properties of wool fibers was investigated in relation to sulfhydryl concentration and disulfide bond decrease. It was shown that only the highest level of modification (200 ? mole SH/g) influences the properties in the Hookean and yield regions. This effect could be assigned partly to the increase in SH content and partly to the decrease in disulfide content. In the post-yield region. the influence of chemical modification was much more pronounced and became evident even at the lower levels of fiber modification. The turn-over point between the yield and post-yield regions underwent a transition at 72° C for the untreated fibers. The transition temperature was found to be strongly dependent upon the SH content, decreasing with increasing sulfhydryl concentration. In the absence of SH groups, the transition temperature was increased to approximately 85° C and was independent of the disulfide content. This transition is discussed in terms of the stability of the disulfide bond, which is closely connected with the probability of an interchange between disulfide bonds and SH groups. It is suggested that the transition indicates the onset of a chemical flow region in the matrix rather than a glass transition as proposed by other workers.