The monoclonal antibody DX5 reacts with CD49b, also known as very late antigen-2 (VLA-2) and a2 integrin. The antibody stains the majority of NK cells and a small subpopulation of T cells in all mouse strains tested (e.g., A/J, AKR, BALB/c, C3H/HeJ, C57BL/6, C57BL/10, C57BR, C58, CBA/Ca, CBA/J, DBA/1, DBA/2, SJL, SWR). Three-color flow cytometry analysis has revealed that most cells that express NK-1.1 (NKR-P1C) also express CD49b. However, small subsets of DX5+NK-1.1- and DX5-NK-1.1+ cells can be found, especially among the CD3+ population of T lymphocytes. NK cells cultured in the presence of IL-2 progressively lose reactivity with DX5 as a consequence of cellular proliferation. Therefore, DX5 can be used to define functionally distinct subsets of murine NK cells. The monoclonal antibody DX5 has not been demonstrated to have activating or blocking activity.