CpG ODN stimulate syngeneic antileukemia activity in vivo. BALB/cJ mice were injected with syngeneic, GFP⁺ ALL via tail vein. On day 7 after injection, mice were randomized to receive either CpG ODN or PBS via intraperitoneal injection every 4 days for 3 doses. A subset of mice was killed at day 7, and blood, bone marrow (BM), spleens, and lymph nodes (LN) were harvested, stained for B220, and analyzed by FACS. (A) Day 7 engraftment. Peripheral blood, spleen, BM, and LN are depicted at day 7, when treatment begins. Percentages of GFP⁺/B220⁺ cells are shown, with significant differences noted only between BM and blood (**P < .01) and BM and LN (*P < .05). (B) Day 28 engraftment. Percentages of GFP⁺/B220⁺ cells in peripheral blood, BM, and spleen after treatment with PBS or CpG ODN in mice killed at day 28. CpG ODN treatment conferred significant protection in blood P < .003, marrow P < .005, and spleen (P < .001). (C) Survival after CpG treatment. The remaining mice were followed with weekly peripheral blood assessments, analyzed by FACS for B220 and GFP, and killed when peripheral blasts were more than 70% or with signs of clinical illness. Kaplan-Meier analysis of mice receiving CpG ODN versus PBS controls followed for 6 months after treatment is depicted (P < .001).