α-Difluoromethylornithine (DFMO) delays orthotopic tumor growth of MDA-MB-231 cells. Athymic mice were injected into the second thoracic mammary fat pad with MDA-MB-231 cells. DFMO treatment (2% per orally) began either contemporaneously (▴, n = 20) or 21 days post injection (▼, n = 3). Mammary fat pad tumor length and width were measured once weekly. Control (●, n = 17) and 21 days post injection (DFMO+21) animals were sacrificed 6 weeks post injection. Day of injection (DFMO 0) animals were euthanized either 6 weeks (n = 10) or 11 weeks (n = 10) post injection.