1. Strains of staphylococci produce diazotizable materials which can be converted to a dye, and the intensity of the color reaction can be quantitated in the same manner as p-aminobenzoic acid. The sulfonamide-resistant strains which we have studied produce more diazotizable substance than non-resistant strains.
2. The development of a color by the diazotizable substance can be inhibited by exposing the substance to a soil bacillus (Mirick) specifically adapted to oxidize PABA.
3. The diazotizable substance produced by staphylococci inhibits the anti-staphylococcic action of sodium sulfathiazole to approximately the same degree as equivalent amounts of pure p-aminobenzoic acid.
4. Two microbiological methods for assaying p-aminobenzoic acid were employed for quantitating the amount of this material produced by staphylococci. In general, the sulfonamide-resistant strains produced more p-aminobenzoic acid than the sulfonamide-sensitive strains. The inconstant results obtained with these biological assays are discussed.