THE observation of Woods1, that the anti-bacterial activity of sulphanilamide is inhibited byp-amino-benzoic acid, has been followed from time to time by observations recording a similar anti-sulphanilamide action for other compounds chemically unrelated top-aminobenzoic acid. Thus Harris and Kohn2have shown thatdl-methionine antagonizes sulphanilamide action. Martin and Fisher3have reported that adenine possesses anti-sulphanilamide activity in mice infected with streptococci comparable with that possessed byp-aminobenzoic acid. Snell and Mitchell4found that adenine, guanine, xanthine and hypoxanthine, as well asdl-methionine, reversed sulphanilamide bacteriostasis of certain lactic acid bacteria, although the results were largely dependent on the particular organism and conditions employed. Anti-sulphanilamide action has also been demonstrated with urethane5.In the course of biological examination of some pyrimidines, we have recently examined 2-amino-pyrimidine-5-carboxylic acid6-an acid bearing a close structural relationship to p-aminobenzoic acid. It displayed no anti-bacterial action against Streptococcus pyogenes in vitro, but possessed distinct sulphanilamide inhibitory powers, although in smaller degree than p-aminobenzoic acid. Thus p-amino-benzoic acid was found to be 2,000 times as effective as 2-aminopyrimidine-5-carboxylic acid in inhibiting sulphanilamide bacteriostasis of Streptococcus pyogenes in vitro.