The induction of crown-galls is inhibited by 5-fluorodesoxyuridine (FUDR). This inhibition can easily be distinguished from the much smaller effect of FUDR on Tumor growth and on general plant development. The inhibition can be reversed completely only by thymidine in higher concentrations. The reversibility is restricted to the first 3-4 days after infection of the plant. The virulence of the bacteria and the wound reaction of the plants are scarcely affected by the corresponding concentrations of FUDR. Thus the primary step in the transformation of a plant cell to a tumor cell can be selectively blocked and presupposes an undisturbed DNA synthesis, i. e. a change or activation in DNA is a part of the primary step in the transformation.