Elvax pellets containing interleukin 1 (IL-1) or 2 (IL-2) were assessed for their ability to alter the growth of transplanted C6 cells in rat. C6 cells were injected into the left caudate of Sprague Dawley rats. At the time of tumor cell injection, or 9 days later, Elvax pellets containing either 50 or 200 units of interleukin were inserted into the dorsal aspect of the caudate. Animals were killed 16 days later and the volume of tumors determined. Results showed that introduction of low-dose IL-2 pellets at time of C6 cell injection virtually prevented establishment of tumors, whereas low-dose IL-1 pellet insertion resulted in nearly a threefold increase in tumor size relative to control preparations. Insertion of high-dose IL-2 pellets 9 days after C6 cell injection decreased tumor growth relative to controls by nearly twofold, low-dose pellets having no effect. These effects appear to be mediated by alterations in host brains, vs interleukin-moderating effects on growth of C6 cells themselves, since (a) no effect of IL-2 on C6 cell growth in vitro could be demonstrated and IL-1 appeared to enhance their proliferation in culture, and (b) IL-1 pellets diminished and IL-2 pellets enhanced infiltration of inflammatory cells into brain when implanted into brain after a “stab” wound.