Mg and S appear equally important in yeast growth. Of the forms of S studied, the sulphate is the most available. Sulphate S is a true nutrient in that it is actually converted into yeast protein and probably, at least in part, into cystine. Cystine, cys-teine, and H2S stimulate yeast growth in a sulphate-free medium up to certain, but low concentrations, above which they retard growth. Cystine also stimulates slightly in a sulphate-containing medium, but H2S retards growth in such a medium even in extremely low concentrations. When yeast grows in a sulphate-free medium containing cystine as the source of S, it converts part of the cystine into new yeast protoplasm and about an equal amount into sulphate left in the medium. Taurocholic acid in very low concentrations retards yeast growth. Cysteinic acid acts intermediate between taurocholic acid and cystine. Taurine is without action in yeast growth in the concentrations here employed.