The PC61 5.3 monoclonal antibody (mAb) reacts with the α chain of the IL-2 receptor (IL-2R). IL-2R is expressed by CD4-CD8- thymocytes, and immature B cells in bone marrow. In addition, both activated B and T cells express CD25, which makes the PC61 5.3 mAb useful as an activation marker. The IL-2R has been implicated to function in thymocyte differentiation and lymphocyte proliferation. PC61 5.3 inhibits binding of IL-2 to both high and low affinity IL-2 receptors. Other applications of the PC61 5.3 mAb include immunoprecipitation, immunostaining for flow cytometry, inhibition of IL-2-dependent proliferation of cloned cytolytic T cells, and IHC of frozen sections.