Carrot suspension cultures resistant to growth inhibition by p-fluorophenylalanine, ethionine. aminoethylcysteine, and 5-methyltryptophan were obtained by sequential selection for resistance to each amino acid analogue. Resistance was increased at least 100-fold for each analogue and the resistance was retained after growth away from the inhibitors for 40 cell doublings. After each selection, the corresponding free natural amino acid was increased and the line resistant to all four analogues showed levels of phenylalanine, methionine, lysine, and tryptophan which were 7, 6, 5, and 32 times that of the parental wild type line, respectively. The total free amino acid level was doubled in this line. Only after selection for 5-methyltryptophan resistance did the anthranilate synthetase show altered feedback sensitivity to tryptophan.