The aim of this study was to investigate the activation ability of CpG oligodeoxynucleotide (CpG ODN) 2216 on the peripheral blood mononuclear cells (PBMNCs) from leukemia patients in remission and the killing effect of activated PBMNCs on K562 cells. PBMNCs obtained from leukemia patients in remission were incubated with CpG ODN 2216. In control group, PBMNCs were incubated with normal saline (NS). The concentrations of cytokines (IFN-gamma, interleukin-12, interleukin-4, interleukin-10) in culture supernatant of PBMNCs from leukemia patients in remission were analyzed by using ELISA kits. The percentages of Th1, Tc1, Th2, Tc2 cells and killed K562 cells were detected by flow cytometry. The results showed that as compared with control group, CpG ODN 2216 induced higher concentrations production of IFN-gamma, IL-12 in supernatant (p < 0.01). There were no differences in IL-4, IL-10 in supernatant as compared with control group (p > 0.05). The percentages of Th1 and Tc1 cells increased significantly after culture with CpG ODN 2216 as compared with control group (p < 0.05). There was no difference between the percentages of Th2 and Tc2 cells in stimulated group and control group. The killing effect of PBMNCs on K562 cells was significantly different between the stimulated group and control group (p < 0.05). It is concluded that CpG ODNs 2216 can induce strong Th1-like immune activation, with the secretion of type-I cytokine and activation of strong CD8(+) T-cell responses. PBMNCs activated by CpG ODNs can more strongly kill k562 cells in vitro.