Effects of polyamines on DNA synthesis were studied in synchronized culture of Nicotiana tabacum L. When DFMO and DFMA, inhibitors of ornithine decarboxylase and arginine decarboxylase, respectively were initially applied to the cells, the polyamine contents were rapidly dropped and [methyl-3H] thymidine incorporation into DNA was markedly reduced during the early stage of culture period. Inhibition of DNA synthesis, however, was partially reversed when these inhibitors were applied simultaneously with putrescine. In addition, exogenous administration of putrescine also increased the DNA synthesis during the all over the culture period. In vitro activity of DNA polymerase from Nicotiana tabacum L. was promoted by increasing concentrations of polyamines in the reaction mixture. Maximal activity was shown at 5 mM putrscine, 0.5 mM spermidine and spermine, respectively. Lack of Mg2+ ion in the reaction buffer resulted in an inhibition of the enzyme activity by about 30%. The inhibition could not be completely reversed by application of polyamines at optimal concentrations. These results suggest that polyamines promote the DNA synthesis in vivo and in vitro by stabilizing the DNA-helix upon binding to negatively charged groups on DNA or increasing the activity of DNA polymerase in Nicotiana tabacum L.