Daclizumab, a humanized antibody directed against the alpha-chain of the interleukin-2 receptor (CD25), has shown efficacy in the prevention of acute rejection following organ transplantation. However, anti-CD25 therapy can be expected to affect not only alloreactive effector T cells, but also CD4(+)CD25(+) regulatory T (Treg) cells that are shown to play an important role in the induction of transplantation tolerance. Therefore, the size and function of the Treg pool in human renal allograft recipients after single-dose daclizumab administration was investigated in this study. Approximately 8 weeks after administration, daclizumab was cleared from the circulation and the Treg population then present appeared not different from that observed before transplantation. Functional analysis revealed that the Treg possessed a normal capacity to suppress mixed lymphocyte reactions in vitro. These data indicate that after daclizumab therapy a Treg population, normal in number and function, is present in the peripheral blood of renal transplant recipients.