The disulphide bonds of two samples of Merino 70s wool were reduced to the extent of 38% and 56% respectively and S-carboxymethylated with iodo (2-14C)-acetate.The disulphide bonds remaining intact after these lust-stage reductions were reduced subsequently and S-earboxy­methylated with non-radioactive iodoacetate. The two major low-sulphur proteins of wool (Com­ponents 7 and 8), three high-sulphur proteins (SCMKB2·A, -8 and-C), and a a<-helical polypeptide fraction were isolated from each of the two 14C-Iabelled wools, and the extents to which the half­cystine residues in all of them were reduced in the first stage determined by their contents of 14C, S-earboxymethylcysteine, cystine and cysteic acid. At both levels of lust-stage reduction the low­sulphur proteins were reduced to a much higher degree than either the high-sulphur proteins or the ae-heJical protein fraction. However, although the percentages of half-cystine residues reduced at the 38% level were very different in the two types of protein, the numbers of half-eystine residues reduced per molecule were approximately the same. This lmding, and other evidence, suggests that the disulphide bonds that are more easily reduced in wool are interchair. disulphide bonds linking low­and high-sulphur proteins in the microfibril-matrix structure of the cortex.