AIM: To investigate the effect of CpG-containing oligodeoxynucleotides (CpG ODN) alone or in combination with the chemotherapeutic agent 5-fluorouracil (5-FU) on tumor growth and whether CpG ODN can reverse the immunosuppression caused by the chemotherapy with 5-FU in murine hepatoma model.METHODS: Hepatoma model was established by subcutaneous inoculation with hepatoma-22 (H22) cells into the right flank of BALB/c mice. Mice with tumor were treated by peritumoral injection of CpG ODN alone or in combination with subcutaneous injection of 5-FU. Tumor size was quantified regularly. Serum levels of IL-12 and IFN-γ in mice were assayed by enzyme-linked immunosorbent assay (ELISA). The lytic capacity of splenic NK cells was tested by lactate dehydrogenase release assay.RESULTS: Peritumoral injection of CpG ODN alone or in combination with subcutaneous injection of 5-FU, and the treatment with 5-FU alone all led to significant inhibition of hepatoma growth. The mean tumor volumes fell by 46.66% in mice injected with CpG ODN, 68.34% in the 5-FU treated mice, and 70.23% in mice treated with the combination of CpG ODN and 5-FU than in controls. There was no significant difference in tumor size between 5-FUtreated mice and mice treated with the combination of 5-FU and CpG ODN (P＞0.05). The serum levels of IL-12and IFN-γ of mice treated with CpG ODN alone (IL-12:464.50±24.37 pg/mL; IFN-γ:134.20±25.76 pg/mL) or with the co-administration of CpG ODN and 5-FU (IL-12:335.83±28.74 pg/mL; IFN-γ:111.00±5.33 pg/mL)were significantly higher than that of controls (IL-12:237.50±45.31 pg/mL; IFN-γ: 56.75±8.22 pg/mL). The production of IL-12 and IFN-γwas suppressed moderately in 5-FU-treated mice (IL-12:166.67±53.22 pg/mL;53.33±16.98 pg/mL) compared to control mice (P＞0.05),whereas the combination of CpG ODN and 5-FU significantly increased the serum levels of IL-12 and IFN-γ compared to 5-FU alone (P＜0.05). The NK cell killing activity in CpG ODNtreated mice (44.04±1.38%) or the mice treated with CpG ODN combined with 5-FU (30.67±1.28%) was significantly potentiated compared to controls (19.22±0.95%, P＜0.05).The co-administration of CpG ODN and 5-FU also significantly enhanced the lytic activity of NK cells when compared with the treatment with 5-FU alone (12.03±1.42%, P＜0.05).CONCLUSION: The present data suggests that CpG ODN used as single therapeutic agent triggers anti-tumor immune response to inhibit the growth of implanted hepatoma and reverses the immunosuppression caused by the chemotherapy with 5-FU.