The Lactobacillus arabinosus 17-5 response curve for anthranilic acid in the absence of tryptophan is characterized by regular increase in growth to about 100 micrograms per 10 milliliter culture, steady decrease to about 500 micrograms, nearly complete inhibition from 1000 to 5000 micrograms, and fair growth above 10,000 micrograms. The inhibition is prevented by low concentrations of indole or tryptophan or by allowing initiation of growth at lower levels of anthranilic acid. Although added at normally inhibitory levels, anthranilic acid is then converted to tryptophan and heavy growth ensues;The inhibition is believed to involve interference with some essential cellular process, possibly a step in the conversion of anthranilic acid to tryptophan;A mutant strain (designated L. arabinosus A) overgrows inhibited cultures in two to three days. This mutant apparently differs from L. arabinosus 17-5 only in its insensitivity to anthranilic acid at levels inhibitory to the latter strain;Five N-substituted anthranilic acid derivatives were tested for inhibition of or participation in tryptophan synthesis. No inhibition was observed, and such tryptophan activity as exists is apparently due to conversion to anthranilic acid. Similar tests of 5-methyl- and 5-chloroanthranilic acids and of o-chloro-, o-bromo-, and o-methylbenzoic acids revealed neither type of activity;Attempts to determine the course of the indole-tryptophan conversion were unsuccessful.