Note:Natural killer (NK) cells can kill tumor and virally infected cells in a manner that is only beginning to be characterized at the molecular level. Results reported in the last few years support the concept that NK cells may display two types of receptors; one triggering NK cell lysis upon recognition of target cell carbohydrate structures, and one inhibiting cytotoxicity when engaged by particular target cell MHC class I molecules. Our laboratory produced a mAb (4LO3311) identifying a murine NK cell surface antigen, and provided evidence that this antigen is an activating receptor. In this work, we established that, in fact, mAb 4LO3311 recognizes the NK2.1 antigen, formerly defined by an NZB anti-BALB/c antiserum. We characterized NK2...