FN50 antigen is the earliest inducible type II cell surface glycoprotein to appear upon ‘in vitro’ activation of T cells, NK cells and B cells. CD69 is undetectable on the plasma membrane of most circulating peripheral blood lymphocytes (PBL). Other cell types, including platelets, neutrophils, eosinophils, and epidermal Langerhans cells, also express the CD69 antigen. CD69 is highly expressed on T cells from inflammatory infiltrates of several human diseases: rheumatoid arthritis, viral hepatitis, autoimmune thyroid disorders. CD69 is constitutively expressed by a subset of medullary mature thymocytes, mantle B cells and certain CD4+ T cells in the germinal centers of normal lymph nodes, platelets, epidermal Langerhans cells.