SUMMARY: Sulphanilamide inhibition of the growth of certain yeasts and yeast- like organisms was overcome in a non-competitive manner by methionine, adenine and histidine, which are therefore regarded as end-products of enzyme systems for which p-aminobenzoic acid is essential. The methionine system was the most sensitive to sulphanilamide, followed in order by those of adenine and histidine. In the presence of methionine, adenine and histidine together, the test organisms were still sensitive to sulphanilamide, implying that p-aminobenzoic acid is con- cerned in still another system, or systems. The nature of these systems is obscure, but the inhibition is not overcome by purines, pyrimidines or pteroylglutamic acid, nor by single amino-acids, except in one case when m-cysteine showed some effect. In another case, a low concentration of choline chloride was effective. Evidence of the vital functions of p-aminobenzoic acid (p-AB) may be obtained by the method of inhibition analysis using sulphanilamide (S-amide) (Shive & Roberts, 1946). The biosynthesis of methionine and of the purine skeleton seems to be dependent on enzyme systems in which p-AB plays an essential role, since methionine and purines overcome S-amide inhibition in a non- competitive manner (Harris & Kohn, 1941; Snell & Mitchell, 1942; Lampen & Jones, 1947). This conclusion is supported by other studies which showed that p-AB can be replaced by purines and amino-acids in the growth of mutant strains of Eschrichia coli (Lampen, Jones & Roepke, 1949) and by adenine, methionine and histidine for the growth of a yeast (Cutts & Rainbow, 1950). In other cases, S-amide inhibition is overcome by relatively high concentra- tions of thymine or by relatively low concentrations of pteroylglutamic acid (PGA). It is therefore concluded that PGA is concerned in the biosynthesis of thymine (Lampen & Jones, 1947; Winkler & de Haan, 1948). As a result of extensive inhibition analysis of Esch. coli, the latter workers found that serine is an S-amide antagonist, and Lampen et al. (1949) showed that other amino- acids have some effect. In the present study, purines, pyrimidines, amino- acids, PGA and choline were tested as antagonists of S-amide bacteriostasis of some yeast and yeast-like organisms, and the results compared with those already obtained by a different technique (Cutts & Rainbow, 1950).