Immunostimulatory CpG oligodeoxynucleotides (ODNs) can enhance the therapeutic effect of monoclonal antibodies (mAbs) by enhancing antibody-dependent cell-mediated cytotoxicity (ADCC). Distinct classes of CpG ODNs have been found recently to stimulate different effector cell populations. We used murine cancer models to explore the role of various effector cell populations in the antitumor activity seen with mAbs combined with CpG ODNs of the A and B classes. In the 38C13 syngeneic murine lymphoma model, both CpG A and CpG B enhanced the efficacy of murine antilymphoma mAb. Depletion of natural killer (NK) cells alone markedly decreased the efficacy of therapy with mAbs plus CpG A. In contrast, depletion of both NK cells and granulocytes was required to decrease the efficacy of mAb plus CpG B. A human (h) Fc γ receptor I (FcγRI)-expressing transgenic (Tg) mouse model was used to explore the role of FcγRI in therapy with mAb and CpG ODN. CpG B induced up-regulation of FcγRI in hFcγRI Tg mice, whereas CpG A did not. In vitro CpG B also enhanced ADCC of HER-2/neu-expressing tumor cells by the FcγRI-directed bispecific antibody MDX-H210 using hFcγRI-positive effector cells. In a solid tumor model, tumor growth was inhibited in Tg mice treated with a combination of MDX-H210 and CpG B. These data suggest that CpG A enhance ADCC largely by activating NK cells. In contrast, other effector cell populations, including granulocytes, contribute to the antitumor activity of CpG B and mAbs. FcγRI plays an important role in this activity.