Rituximab (IDEC-C2B8, Mabthera(R)) is a chimeric (human-mouse) monoclonal antibody (MoAb) against the B-cell specific CD20-antigen. It has been used for the clinical treatment of non-Hodgkin's lymphomas, but variable clinical results suggest that some lymphoma cells remain resistant. In the present study we have evaluated the relative efficiencies of humoral and cell-mediated effector mechanisms complement-dependent cytotoxicity (CDC), antibody-(ADCC), complement-(CDCC) dependent cellular cytotoxicity and apoptosis on lymphoma cell killing by rituximab. Rituximab activated the cytolytic complement (C) cascade and induced a strong CDC, but the rituximab-triggered ADCC and CDCC were relatively ineffective. The CDC was strongly enhanced by antibodies against the C inhibitor CD59 (protectin). Neutralization of CD55 (DAF) and CD46 (MCP) had a similar but weaker effect. Rituximab also induced apoptosis but in a cell line-dependent fashion. The results strongly emphasize the role of direct CDC as the major, fast and efficient effector mechanism of rituximab. In the immunotherapeutic treatment of B-cell lymphomas, it is important to consider the role of C-regulatory proteins as an escape mechanism of the malignant cells. Our results suggest that the effect of rituximab therapy could be enhanced by combining it with neutralization of CD59.