CD40 and CD20 are expressed in several B-cell malignancies and represent attractive targets for therapeutic intervention. The anti-CD20 monoclonal antibody, rituximab, is an approved drug for the treatment of non-Hodgkin’s lymphomas. however, the existence of patients with rituximab-resistant disease limits its clinical utility. We have previously reported that the novel, highly potent, fully human antagonistic anti-CD40 monoclonal antibody, CHIR-12.12, generated from XenoMouse® mice (Abgenix, Inc) has greater anti-tumor activity than rituximab in both rituximab-responsive and rituximab-resistant human NHL models. In this study, we evaluated the potential therapeutic application of combining CHIR-12.12 and rituximab for the treatment of NHL. Namalwa is a Burkitt’s lymphoma cell line that gives rise to aggressive rituximab-resistant tumors when implanted in nude mice. Direct treatment of these tumor cells with CHIR-12.12 or rituximab in culture does not affect cell growth when compared to treatment with an isotype control antibody. Although Namalwa cells express more CD20 than CD40 (average of 10,059 CD20 and 3,138 CD40 molecules per cell respectively, P=0.05), when tested for in vitro ADCC killing using human NK cells as effectors, CHIR-12.12 mediated stronger target cell lysis than rituximab (31.43% vs. 14.15%, P<0.0001). Adding CHIR-12.12 and rituximab together did not enhance the in vitro ADCC killing. When CHIR-12.12 and rituximab were tested in a subcutaneous Namalwa xenograft model, CHIR-12.12 alone caused 60% tumor growth inhibition (P=0.028) whereas rituximab alone at 10 and 20 mg/kg did not inhibit Namalwa tumor growth. When tumor-bearing mice were administered rituximab at 10 mg/kg plus CHIR-12.12 at 5 or 10 mg/kg, synergistic anti-tumor activity was observed in a CHIR-12.12 dose-dependent manner. The mean tumor volume reduction in combination groups is 77% with CHIR-12.12 at 5 mg/kg (P=0.0037) and 83% with CHIR-12.12 at 10 mg/kg (P=0.0018), respectively. The potential interaction between CD40 and CD20 molecules was evaluated in vitro by treating the tumor cells with CHIR-12.12 and assessing the change in CD20 expression and vice versa. The result showed no augmentation of one antigen expression by treating the tumor cells with the other antibody. The mechanism of anti-tumor synergy observed in this combination is under evaluation. Taken together, these data suggest that the combination therapy with anti-CD40 CHIR-12.12 and rituximab has the potential to improve patient outcome in B-cell malignancies co-expressing CD20 and CD40 antigens and support the further development of CHIR-12.12 antibody for treatment of B-cell malignancies.