Auxin autotrophy was studied in cultured carrot (Daucus carota L.), tobacco (Nicotiana tabacum L.), and potato (Solanum tuberosum L.) cell lines. Of 10 carrot lines resistant to 5-methyltryptophan (5MT), which accumulate free tryptophan (trp) because of an altered control enzyme, 5 were auxinautotrophic while the normal parent line was not. Carrot lines selected from the same parent line as resistant to other amino-acid analogs were not auxinautotrophic, like the parent. The only 5MT-resistant potato line studied was also auxin-autotrophic while the normal line was only partially auxin-autotrophic. The tobacco lines which accumulated free trp were not auxin-autotrophic, and no auxin-autotrophic tobacco lines were selected by screening for growth in medium lacking 2,4-dichlorophenoxyacetic acid (2,4-D). Several auxin-autotrophic carrot and potato lines were selected from the normal lines and none of these lines were resistant to 5MT. Length of time in culture and difficulty in selecting auxin-autotrophic lines were correlated on the 3 normal carrot lines studied. The addition of trp or indole to the culture medium would partially alleviate the auxin requirement of the normal lines studied. 2,4-D (0.4 mg/l) stimulated the growth of all auxin-autotrophic carrot lines.