The Jak inhibitor CP-690,550 inhibits alloreactivity and is currently being investigated for prevention of allograft rejection after transplantation. In this study, we examined the effect of CP-690,550 on IL-2-mediated Jak/STAT5 phosphorylation by CD4(+)CD25(bright)FoxP3(+)CD127(-/low) T cells (Treg) and CD4(+)CD25(neg) effector T cells (Teff) in kidney transplant (KTx) patients. Phosphospecific flow cytometry was used to study the effect of CP-690,550 on IL-2-induced intracellular STAT5-phosphorylation. IL-2-induced phosphorylation of STAT5 (P-STAT5) in both Treg and Teff, which was significantly higher for CD4(+)CD25(bright) Treg (increased by 71%, mean) than for CD4(+)CD25(neg) Teff (increased by 42%). In the presence of 100 ng/mL CP-690,550, a clinically relevant exposure, IL-2-induced P-STAT5 was partially inhibited in CD4(+)CD25(bright)Treg (% inhibition; 51%), while almost completely blocked in Teff (%inhibition; 84%, p = 0.03). The IC(50) was 2-3 times higher for Treg (104 ng/mL) than for Teff (40 ng/mL, p = 0.02). In the presence of CP-690,550, Treg exhibited additional suppressive activities on the alloactivated proliferation of Teff (56%, mean). In addition, CD4(+)CD25(bright) Treg from KTx-patients receiving CP-690,550 vigorously suppressed the proliferation of Teff (87%, mean). Our findings show that CP-690,550 effectively inhibits Teff function but preserves the suppressive activity of CD4(+)CD25(bright) regulatory T cells.