CD40 is expressed on chronic lymphocytic leukemia (CLL) cells, and CD40 activation leads to signaling critical for cell survival and proliferation. We have previously described a novel, fully human IgG1 anti-CD40 antagonistic monoclonal antibody, CHIR-12.12, generated in XenoMouse® mice (Abgenix, Inc.), and have demonstrated that it inhibits normal human B cell proliferation and survival and mediates potent antibody-dependent cellular cytotoxicity (ADCC) against primary CLL and non-Hodgkin’s lymphoma cells. In this study, we examined the ability of CHIR-12.12 to modulate cytokine production by primary CLL cells and compared the ADCC activity of CHIR-12.12 with rituximab against primary CLL cells. Primary CLL cells stimulated with CD40L produced a variety of cytokines, including IL-10, TNF-α , IL-8, GM-CSF, IL-6, MCP-1, and MIP-1β. Addition of CHIR-12.12 to primary CLL cells inhibited CD40L-mediated production of these cytokines. Cytokine production by primary CLL cells cultured with CHIR-12.12 alone in the absence of CD40L did not exceed levels produced by CLL cells cultured in medium. These data suggest that CHIR-12.12 is a potent antagonist for CD40L-mediated cytokine production by primary CLL cells and shows no agonistic activity by itself. We next compared the relative ADCC activity of CHIR-12.12 and rituximab against ex vivo primary CLL cells from 8 patients. CHIR-12.12 exhibited greater ADCC than rituximab against CLL cells from all patients. The average percent of maximum lysis by CHIR-12.12 and rituximab were 49 ± 16% and 31 ± 14%, respectively. CHIR-12.12 was greater than 10-fold more potent than rituximab, as measured by ED50 values (14.1 pM versus 155.5 pM, respectively). Quantitative CD20 and CD40 density on CLL cells and the degree of antibody internalization were investigated as potential reasons for the difference in ADCC activity. The greater ADCC potency and efficacy of CHIR-12.12 was not dependent on a higher density of cell surface CD40 molecules, as there were 1.3 to 14-fold higher numbers of CD20 than CD40 molecules on the cell surface. Antibody internalization studies using primary CLL cells conducted by flow cytometry and confocal microscopy show that upon binding to CD40 at 37°C, CHIR-12.12 remains uniformly distributed on the cell surface, even after 3 hours. In contrast, after binding at 37°C, rituximab is redistributed into caps and internalized. These data suggest that the potent ADCC activity of CHIR-12.12 may be partly related to its ability to remain on the surface of target cells uniformly, allowing optimal interaction with effector cells. Taken together, these results suggest that CHIR-12.12 may be effective at mediating potent ADCC against CLL cells in vivo. CHIR-12.12 is currently in Phase I trials for B-cell malignancies.