Evidence is presented for a mechanism of setting in which the know n conformational changes in the poly peptide structure involve disulfide bond rearrangement via thiol disul fide interchange. The new arrangement of disulfide bonds helps to stabilize the new structure if reversion is inhibited by preventing further interchange. Thiol and disulfide analyses of wools before and after setting support this hypothesis. We have found no evidence for the formation of stabilizing cross-links other than disulfide bonds. This hypothesis accounts for the fact that the rate of setting in boiling water is enhanced by the addition of reducing agents, such as bisulfite or thioglycollate, which generate extra thiol groups It also explains why wool in which the natural thiol groups have been destroyed cannot be set in water but can be set using reducing agents. The occurrence of thiol disulfide interchange has also been demonstrated during the release of set. Release is accelerated by thiol-producing reagents and retarded by con ditions which inhibit interchange. A higher level of set may therefore be obtained by blocking or oxidizing free thiol groups in the set fibers, particularly after short setting times. Cross-linking reagents are no more effective than monofunctional blocking reagents for this purpose.