We have described the modulation of four auxin-regulated genes during the growth cycle of suspension-cultured tobacco (Nicotiana tabacum [L.] var White Burley) cells. The genes were transiently expressed 2 to 8 h after transfer of stationary phase cells to fresh medium, during the transition from the quiescent phase of cells leaving the mitotic cycle to the synthesis phase of the cell cycle. After this transient induction, the cells showed a decreased sensitivity to auxin. Although the expression pattern suggests that induction of these genes might be important for cell division, over-production of antisense mRNA for one of these genes (pCNT103) did not influence cell division in transgenic tobacco cells. Furthermore, stimuli such as salicylic acid were capable of inducing gene expression but were unable to restore cell division. Although these data do not conclusively exclude a role for these genes in cell division, their significance in this process is discussed in view of their homology with other auxin-induced genes and in view of the specificity of hormone-induced early responses.