DX5 mAb is a useful reagent because it stains NK cells from all mouse strains examined. We have identified the molecule recognized by DX5 mAb by using a retrovirus-mediated expression cloning system. A 5-kb cDNA encoding a protein that is reactive with the DX5 mAb was isolated from a NK cell cDNA library, and this molecule was identical with CD49b (very late Ag-2, α 2 integrin). The DX5 mAb reacted with transfectants expressing CD49b, and binding of DX5 to the NK cells and CD49b transfectants was blocked in the presence of other anti-CD49b mAbs. When NK1. 1+ NK cells were cultured with IL-2, they progressively lost reactivity with DX5 mAb as a consequence of cellular proliferation. Cytotoxicity mediated by the DX5+ NK cells was dramatically higher as compared with DX5− NK cells. Therefore, DX5 mAb recognizes CD49b and can be used to define functionally distinct subsets of NK cells.