Long-term anti-tumor immunity. Mice that rejected the initial i.d. tumor derived from CT-26 cells were re-challenged intradermally with 5 × 106 parental cells 8 months after the initial tumor eradication (9 months after the initial tumor inoculation). Tumor growth was observed after 30 days (upper: a control mouse; lower: a mouse that previously eradiated the initial tumor). n = 2.