IAA content was measured in a cloned, crown-gall [Agrobacterium tumefaciens] teratoma line of N. tabacum L. cv. Turkish by a highly specific and sensitive radioimmunoassay. This tissue line, which does not require auxin for continuous growth in culture, exhibits 2 phases of growth. During the first phase, which lasts about 2 wk after subculturing, growth is exponential on a fresh weight basis and the content of IAA is about 10-7-10-6 mol .cntdot. kg-1, but variable. During the 2nd phase, growth rate declines gradually and the IAA content of the tissue drops dramatically; however, this drop does not result from a net loss of auxin by the tissue. The growth rate during the exponential phase was not correlated with the IAA content of the tissue, but was strongly correlated with the IAA content of the inoculum. Rapidly growing leaves of Turkish tobacco have roughly the same IAA content as cultured teratoma tissues. Moreover, both tissues exhibited a similar relationship between auxin content expressed per leaf or per tissue explant and growth. These findings do not support the hypothesis that the autonomous growth of plant tumors results from an abnormally high content of auxin in the tissue.