Multiple mechanisms of B lymphoma cell killing by CIK cultures in the presence of anti-CD20 mAbs and human serum. CIK cells kill B lymphoma target by an HLA-unrestricted mechanism (natural cytotoxicity, NC). In the presence of anti-CD20 mAbs, complement-dependent cytotoxicity (CDC) may take place, as well as antibody-dependent cellular cytotoxicity (ADCC), by activated NK cells present in CIK cultures. NK activation and ADCC induced by rituximab, but not by GA101 coated target cells, are inhibited by complement fragments deposited on the antibody.