We have studied the mechanism of auxin autonomy in tobacco (Nicotiana tabacum L.) crowngall tissues transformed by the auxin-mutant (tms-) A66 strain of Agrobacterium tumefaciens. Normally, tins-tobacco tumor tissues require the formation of shoots to exhibit auxin-independent growth in culture. We have isolated from tms-tobacco cells several stable variants that are fully hormone-independent and grow rapidly as friable, unorganized tissues, thus mimicking the growth and morphology of tms § tobacco cells that produce high levels of auxin. However, none of the variants contained the high levels of auxin found in tms + tumor cells. The variants could be divided into two classes with respect to their response to applied auxin. The first class was highly sensitive to applied auxin: low concentrations (1 IxM) of a-naphthaleneacetic acid (NAA) severely inhibited growth and markedly stimulated the accumulation of the ethylene precursor, 1-aminocyelopropane-l-carboxylic acid (ACC). The second class of variants showed a low sensitivity to applied auxin: growth was promoted by concentrations of NAA up to 10 ~tM, and growth inhibition and high ACC levels were observed only at high NAA concentrations (100 I~M). Unorganized variants with low auxin sensitivity were also isolated from a variant line with high auxin sensitivity. The isolation of tumor cells that exhibited the growth phenotype of tms § cells while retaining the low auxin content and low auxin sensitivity of tms-cells indicates that full hormone autonomy, characteristic of wild-type crown-gall tumors,