Various numbers of spleen cells from specifically immunized mice were mixed with constant numbers of target tumor cells, and were inoculated subcutaneously into thymectomized, x-irradiated recipients. Small numbers of admixed immune spleen cells produced a statistically significant, and reproducible, acceleration of tumor growth in the inoculum as compared with controls of either nonimmune spleen cells or spleen cells from animals immune to a different, non-cross-reacting, tumor. Larger. numbers of specifically immune spleen cells, however, produced inhibition of tumor growth. These data imply that the normal immune reaction may have a dual function in relation to neoplasia: (i) stimulation of tumor growth, early in the course of the disease, or whenever the immune reaction is minimal; (ii) inhibition of tumor growth at other times.