Disease progression of feline immunodeficiency virus (FIV) infection is characterized by up-regulation of B7. 1 and B7. 2 costimulatory molecules and their ligand CTLA4 on CD4+ and CD8+ T cells. The CD4+ CTLA4+ B7+ phenotype described in FIV+ cats is reminiscent of CD4+ CD25+ CTLA4+ cells, a phenotype described for immunosuppressive T regulatory (Treg) cells. In the present study, we describe the phenotypic and functional characteristics of CD4+ CD25+ T cells in PBMC and lymph nodes (LN) of FIV+ and control cats. Similar to Treg cells, feline CD4+ CD25+ but not CD4+ CD25− T cells directly isolated from LN of FIV+ cats do not produce IL-2 and fail to proliferate in response to mitogen stimulation. Unstimulated CD4+ CD25+ T cells from FIV+ cats significantly suppress the proliferative response and the IL-2 production of Con A-stimulated autologous CD4+ CD25− T cells compared with unstimulated CD4+ CD25+ T cells from FIV− cats. Flow-cytometric analysis confirmed the apparent activation phenotype of the CD4+ CD25+ cells in LN of chronically FIV+ cats, because these cells showed significant up-regulation of expression of costimulatory molecules B7. 1, B7. 2, and CTLA4. These FIV-activated, anergic, immunosuppressive CD25+ CTLA4+ B7+ CD4+ Treg-like cells may contribute to the progressive loss of T cell immune function that is characteristic of FIV infection.