Auxin (11 gM 0~-naphthaleneacetic acid) and cytokinin (1.4 gM kinetin) regulate cytokinin accumulation by cytokinin-requiring (C-) and cytokinin-autotrophic (C +) lines of Havana 425 tobacco (Nicotiana tabacum L.) tissues. No transzeatin riboside (ZR) ( < 0.5 pmol. g-1 fresh weight) was detected in six C and nine C + lines grown for 14 d on auxin+cytokinin and auxin medium, respectively. C + lines, but not C-lines accumulated ZR (1.9-5.1 pmol-g-1 fresh weight) when incubated on hormone-free medium but both lines accumulated ZR when incubated on kinetin medium. Therefore, it appears that kinetin treatment can induce ZR accumulation and that this accumulation is blocked by auxin treatment. Similar effects were obtained with some lines of cells autotrophic for both auxin and cytokinin. Tobacco plants carrying the dominant Habituated leaf-1 allele (Hl-1) differ from wild-type plants in that leafderived tissues in culture exhibit a C + phenotype. No differences in ZR content were found in C + leaf tissues from Hl-1/Hl-1 plants and C + tissues that arise epigenetically in wild-type plants. This indicates that the H-1 allele does not act to induce overproduction of ZR. The Hl-1 allele is known to have oncogenic functions similar to the isopentenyl transferase (ipt) locus of the Ti plasmid. Although HI-1/HI-1 cells transformed with Ti plasmids defective at the ipt locus are tumorigenic and hormone-autotrophic in culture, they contain low levels of ZR typical of non-transformed Hl-1/Hl-! cells. Therefore, the high levels of ZR characteristic of cells transformed with wild-type Ti plasmids are not necessary for expression of the tumor phenotype.