The H1.2F3 monoclonal antibody reacts with mouse CD69, also known as Very Early Activation (VEA) antigen, which is inducibly expressed on T cells, B cells, NK cells, monocytes, and neutrophils. CD69 acts as a costimulatory molecule for T cell activation and proliferation and plays a role in the activation and differentiantion of a wide variety of hematopoeitic cells. The H1.2F3 mAb induces T cell proliferation in the presence of PMA and FcR-bearing accessory cells.