The IL-2 receptor (IL-2R) exists in three alternative forms made up from the individual components of CD25, CD122, and CD132. CD25 represents the low affinity alpha chain of the IL-2R, a type I transmembrane glycoprotein containing two CCP domains. It is rich in O-linked carbohydrates and has a short cytoplasmic tail.2 CD25 is expressed on activated T cells, B cells, NK cells and monocytes of all mouse strains tested.1, 3, 4 It is transiently expressed at a low level during B-cell development in bone marrow on the CD45R/B220low TdT- sIg- pre-B II and CD45R/B220low TdT- sIgM+sIgD- immature B cells, but not on the CD45R/B220low TdT+ sIg- pro-B/pre-B I stage nor on CD45R/B220high TdT- sIgM+sIgD+ mature B cells.5, 6 CD25 is expressed at a higher level during very early T-cell development in fetal and adult thymus.6, 7 While the antibody does not block the binding of IL-2 to CD25, when used in combination with C2275-82 it results in higher levels of inhibition of IL-2-driven proliferation than either of these two monoclonal antibodies alone.3, 8, 9