The CD25 antigen (IL-2Rα chain, Tac or p55) is a single-chain glycoprotein with a molecular weight of 55 kDa which is the low affinity receptor for IL-2 (IL-2Rα or IL-2R1). In conjunction with CD122 (IL-2Rβ, p75) and CD132 (IL-2Rγ, p64), CD25 antigen forms the high-affinity IL-2R complex. CD25 is highly expressed on regulatory CD4-positive T lymphocytes and undetected on resting CD8 positive lymphocytes. However, all activated T lymphocytes express the CD25 protein. CD25 is also found on a subset of B cells (CD20 positive), on activated monocytes and macrophages, and on T cell clones. It is also expressed on NK cells enriched from peripheral blood lymphocytes by culturing in the presence of IL-2. It is absent from thymocytes, granulocytes, monocytes, resting NK cells, platelets and erythrocytes.