Dilution of quiescent L1210-DFMOr (difluoromethylornithine-resistant) cells in fresh medium containing serum led to the induction of ornithine decarboxylase (ODC) and to the expression of its mRNA, as determined by a sensitive solution-hybridization-RNase-protection assay. Addition of the chelating agent diethylenetriaminepentaacetic acid (DTPA) at seeding time caused an inhibition of the induction of ODC activity by up to 90%, and only Zn2+ of the bivalent metal ions tested was effective in reversing this effect. The inhibition of the induction of ODC activity was accompanied by a marked decrease, prevented by Zn2+ supplementation, of the accumulation of immunoreactive ODC protein and ODC mRNA. DTPA treatment also caused a slight acceleration of ODC turnover. These results indicate that a restricted Zn2+ availability in L1210-DFMOr cells impairs ODC induction remarkably, mainly by affecting the expression of the messenger.