Natural killer (NK) cells represent a potent subset of lymphocytes for targeting and lysing tumor cells. In contrast to T lymphocytes, they do not need to be preactivated in vitro because they constitutively express cytolytic functions against a number of different targets (1 ,2 ). Their inherent cytolytic activity can be stimulated via the FcγIIIA receptor (CD16), which is expressed on the surface of NK cells, macrophages, and activated monocytes (3 ,4 ). Bispecific antibodies binding to both CD16 and a tumorassociated antigen are therefore of great interest as potential reagents for cancer immunotherapy.
