1. 1. Daucus carota (carrot) cell lines were selected which were resistant to growth inhibition by dl-5-methyltryptophan. The resistant cells appeared with a frequency of about 1 in 3.6·10 6 cells, the trait was usually stable and the cells were also resistant to growth inhibition by several other tryptophan analogs. 2. 2. The anthranilate synthetase activity in gel-filtered crude cell extracts from both normal and resistant cells required Mg 2+ and l-glutamine and to the extent of about 10%, NH 4Cl could substitute for l-glutamine. Co 2+ or Mn 2+ could substitute for Mg 2+ while Hg 2+, Pb 2+, Zn 2+ and p-chloromercuribenzoate were potent inhibitors of the reaction. Cu 2+, Fe 2+, Ba 2+, Ca 2+ and iodoacetamide were not very inhibitory. 3. 3. The anthranilate synthetase activity was 50% inhibited by 3.3 μM l-tryptophan and 5.4 μM dl-5-methyltryptophan, but the enzyme from the resistant cells required about a 5-fold higher concentration of either inhibitor to achieve equal inhibition. This inhibition was neither competitive or noncompetitive with chorismate, but was mixed and was noncompetitive with both l-glutamine and Mg 2+. 4. 4. The enzyme from the resistant cell line differed from the normal cell enzyme by: (A) Being more resistant to inhibition by l-tryptophan, dl-5-methyltryptophan and certain other tryptophan analogs, (B) having a higher specific activity, (C) having a lower chorismate K m (26 versus 46 μM) and (D) having lower Mg 2+ K m (36 versus 101 μM). These factors, especially the resistance of the enzyme to inhibition by dl-5-methyltryptophan, could explain the resistance of the cells to dl-5-methyltryptophan. 5. 5. The less sensitive feedback control also allowed the free l-tryptophan level to be about 27-fold higher than normal.