The in vivo grade recombinant anti-mouse CD25 monoclonal antibodies are based on the rat monoclonal antibody anti-CD25 (mouse) with the clone number of PC61, PC61.5 or PC61.5.3. The rat IgG1 lamda monoclonal antibody has been validated and reported for use in immunoprecipitation (IP), immunohistochemistry (IHC), Flow Cytometry (FC), and various functional assays, such as in vivo depletion of CD25+CD4+ Treg cells, in vitro blocking of IL-2 binding to low- and high-affinity receptors, and growth inhibition of IL-2-dependent T-cell lines. Expression of CD25, together with CD4 and FOXP3, is considered a phenotypic signature for Treg cells. In vivo depletion of mouse CD25(+)CD4(+)FoxP3(+) Treg cells using the rat PC61 monoclonal antibody is widely used to characterize the Treg function. The PC61 monoclonal antibody specifically binds to an epitope of CD25 which is distinct from the IL-2 binding site and from those recognized by the 3C7 and 7D4 monoclonal antibodies. By doing so, the antibody blocks binding of IL-2 to CD25.