SUMMARY A study of the accumulation by a brewer's yeast of six sulphur-containing amino acids labelled with sulphur-35 showed several distinct patterns of uptake. L-Methionine was accumulated rapidly and completely from the medium. L-cysteine was taken up slowly, and L-cystine not at all. DL-Ethionine accumulation was incomplete. S-Methyl-L-cysteine and S-ethyl-L-cysteine were taken up rapidly for a short period, and part of the sulphur of these compounds was then released into the medium. Accumulation of the sulphur amino acids was inhibited by certain other amino acids with a close structural relationship to them and when present in high concentration. Cysteine uptake was enhanced by the presence of reduced glutathione. These effects on accumulation were shown to be primarily changes in the transport of the sulphur amino acids into the yeast, although subsequent incorporation into protein may also be affected.