The levels of non-protein sulphydryls, protein sulphydryls and protein-bound sulphydryls were measured in the separated nuclei and cortices from individual normal and cataractous lenses. As the colour of the lens nucleus increases, there is a progressive decrease in the levels of non-protein sulphydryl and protein sulphydryl. In the nuclei of advanced cataractous lenses, these decrease to 10% and 3%, respectively, of the levels found in the normal nucleus. Similar, but smaller, changes take place in the cortex. The level of protein-bound sulphydryl increases in the early stages of cataract formation but thereafter remains constant in both the cortex and nucleus. In the advanced cataractous lens, the protein bound sulphydryl accounts for only about 2% of the loss of protein sulphydryl. There appear to be no intermolecular disulphide bonds in the urea-soluble proteins of the lens but some may be present in the urea-insoluble proteins.