A new approach is described for the use of cytostatic drugs as biological response modifiers (BRM's). Under restricted conditions, strong potentiation of T-effector function can be obtained through preferential elimination of suppressor cells. Recent studies from our group have demonstrated that such conditions are fulfilled when low dosages of certain drugs are injected at a site of low antigenic stimulation. Furthermore, local injection of cytostatic drugs not only enhances the development of cell-mediated immunity in non-sensitized animals, but also facilitates the reversal of existing immunological tolerance. These data shed new light on the tumor regression and induction of tumor immunity observed upon intralesional chemotherapy in experimental tumor models.