1. A soil bacillus has been isolated capable of developing specific adaptive enzymes which oxidize PABA apparently to carbon dioxide, water, and ammonia. A few related compounds which, however, do not give the diazo reaction are similarly attacked.
2. This bacillus is also capable of developing analogous but apparently independent adaptive enzymes specifically directed against anthranilic acid, the ortho isomer of PABA.
3. Sulfapyridine is bacteriostatic for this bacillus and this effect is inhibited as in the case of other bacterial species by PABA. Other effects of the sulfonamide drugs and sulfanilic acid upon the specific oxidative enzymes have been described.
4. The specific adaptive enzymes of this soil bacillus may be used for the identification of as little as 10 gamma of either PABA or OABA. A method is described for carrying out these determinations.
5. The possible advantages of this enzymatic method as compared with the more sensitive but less specific techniques which measure the activity of PABA as a growth factor for certain bacterial species have been discussed.