Both complement and antibody-dependent cellular cytotoxicity (ADCC) contribute to the clinical efficacy of anti-CD20 monoclonal antibody (mAb) therapy. Paradoxically, the C3b component of complement can block interaction between mAb and natural killer (NK) cells. The present study compared the effect of complement on the ability of two anti-CD20 mAbs, rituximab and GA101, to activate NK cells and mediate ADCC. Complement blocked adherence of NK cells to rituximab, but had little effect on NK binding to GA101. Target cells coated with rituximab or GA101 were able to activate NK cells in the absence of serum. Complement in serum blocked NK activation induced by rituximab, but not GA101. Complement blocked rituximab-induced NK-cell mediated ADCC, but not GA101-induced ADCC. These results demonstrate that the decreased ability of GA101 to fix complement relative to rituximab results in an enhanced ability of GA101 to bind to NK cells, activate NK cells and induce ADCC when serum is present.