Phosphorothioate oligodeoxynucleotides with CpG motifs (CpG-ODNs) activate various immune cell subsets and induce production of numerous cytokines. To evaluate whether CpG-ODNs can induce rejection of established malignant tumor, A/J mice were challenged by the s.c. implantation of a syngenic neuroblastoma cell line (neuro2a) and subsequently injected with CpG-ODNs in the vicinity of the tumor. Daily injections of 10 microg CpG-ODNs for 15 days seemed to be the most potent regimen, leading to the eradication of 5-mm-diameter tumors in one-half of the animals and a significant tumor growth inhibition when compared with controls (88% reduction volume; P<0.001). CpG-ODN-cured animals were further protected against a new tumor challenge. The antitumoral effect of CpG-ODNs was dependent on CpG motifs, and natural killer cells seemed to play a critical role in tumor rejection. We conclude that immunostimulatory CpG-ODNs may induce the rejection of established tumors and warrant further evaluation as a potential immunotherapeutic agent.