The compounds studied were 1-cystine disulphoxide (-SO-SO-), 1-cysteine sulphinic acid (-S2OH), and 1-cysteine sulphenic acid (-SOH), the last being fed as S-(guanylthio)-cysteine hydrochloride, H2N (HN=)C-S-S-CH2-CH(NH2)-COOH, which gives rise to the SOH group on hydrolysis, splitting into the sulphenic acid and thio-urea. This latter as well as its disulphide, dithio-formamidine, was employed as control. The results showed that 1-cystine disulphoxide produced a growth curve similar to that of cystine, S-(guanylthio)-cysteine produced a slight but definite increase in growth, while the sulphenic acid, as well as the two control compounds, produced no growth and showed no toxic effects, their curves being identical with that obtained on the basic diet.