CD40 is expressed on most B cell malignancies including multiple myeloma and represents an attractive target for antibody therapy. We have generated a novel, highly potent, fully human antagonistic anti-CD40 monoclonal antibody, CHIR-12.12, using XenoMouse® mice (Abgenix, Inc). The antibody can mediate anti-tumor activity potentially by at least two mechanisms: CHIR-12.12 can block CD40-ligand mediated survival signals and it can lyse tumor cells by antibody-dependent cellular cytotoxicity (ADCC). We have previously reported that CHIR-12.12 mediates stronger killing of CD40- and CD20-expressing lymphoma cells than rituximab by ADCC in vitro and significantly inhibits the growth of both rituximab-responsive and rituximab-resistant human lymphoma xenografts in vivo. In this study, we examined in vitro and in vivo efficacy of CHIR-12.12 against human multiple myeloma. The human MM cell line IM-9, which expresses both CD40 and CD20, the target antigen for CHIR-12.12 and rituximab respectively was used for the study. CHIR-12.12 induced lysis of target tumor cells by ADCC in a dose dependent manner reaching maximum cell lysis at 0.1ug/ml concentration. The maximum specific lysis of IM-9 cells by CHIR-12.12 was greater than the lysis induced by rituximab (64% vs 45 %, n=3, p<0.01). In addition, the EC50 of CHIR-12.12 was on average 5.9 picomolar, which was 10-fold lower than the EC50 of rituximab. Greater ADCC by CHIR-12.12 was not due to higher density of CD40 molecules on the target tumor cells compared to CD20 molecules. IM-9 cells expressed 35590 ±8858 CD40 molecules compared to 93783 ± 2247 CD20 molecules. The in vivo CHIR-12.12 efficacy was then evaluated in IM-9 xenograft model. In an un-staged conditional survival model, where treatment began one day after intravenous inoculation of IM-9 tumor cells, CHIR-12.12 significantly prolonged the survival of tumor-bearing mice in a dose-dependent manner with 60% survival in the 0.1 mg/kg CHIR-12.12 treated group and 80% survival in the 1 and 10 mg/kg groups respectively on day 56 (Log Rank Test: P<0.01 and P<0.001, respectively). All animals in the control IgG1 and bortezomib treated groups were terminated between day 18 and day 26 due to severe disease related to tumor development (i.e., hind limb paralysis and significant body weight loss). In a staged subcutaneous model, where treatment began once the tumor volume was 150–200mm3, CHIR-12.12 administered weekly at 0.1, 1 and 10 mg/kg significantly inhibited tumor growth with a tumor volume reduction of 17% (P>0.05), 34% (P<0.01) and 44% (P<0.001) respectively. Bortezomib, when tested at 0.5 mg/kg twice a week did not inhibit tumor growth. At the maximally tolerated dose (MTD) of 1 mg/kg twice a week, bortezomib inhibited tumor growth by 30% (P<0.01). Taken together, these data demonstrate that the anti-CD40 mAb CHIR-12.12 has potent activity against human multiple myeloma in vitro and xenograft models in vivo.