The influence of the cytostatic drug Ledakrin on immunologic induction of PF and RF cells was studied. In low doses, the drug had a negligibly small effect on the numbers of PFC and RFC, but in high doses nearly entirely prevented their appearance. Cells producing IgM were more susceptible to this action than IgG-producing cells. Absence of a linear relation between the dose of the drug and its action was noteworthy. Drug-induced immunosuppression occured in "all-or-none" fashion. The possibility of oncostatic effect without interfering with the immunologic system is discussed.