CD4+ CD25+ T regulatory (T R) cells are an important regulatory component of the adaptive immune system that limit autoreactive T cell responses in various models of autoimmunity. This knowledge was generated by previous studies from our lab and others using T R cell supplementation and depletion. Contrary to dogma, we report here that injection of anti-CD25 mAb results in the functional inactivation, not depletion, of T R cells, resulting in exacerbated autoimmune disease. Supporting this, mice receiving anti-CD25 mAb treatment display significantly lower numbers of CD4+ CD25+ T cells but no change in the number of CD4+ FoxP3+ T R cells. In addition, anti-CD25 mAb treatment fails to both reduce the number of Thy1. 1+ congenic CD4+ CD25+ T R cells or alter levels of CD25 mRNA expression in treatment recipients. Taken together, these findings have far-reaching implications for the interpretation of all previous studies forming conclusions about CD4+ CD25+ T R cell depletion in vivo.