CD4+CD25+ regulatory T cells (Treg cells) control immune responsiveness to a large variety of antigens. The isolation and therapeutic manipulation of Treg cells requires the use of reliable surface receptors that are selectively up-regulated in Treg cells. On the basis of global gene expression studies, we identified neuropilin-1 (Nrp1) as a specific surface marker for CD4+CD25+ Treg cells. Nrp1, a receptor involved in axon guidance, angiogenesis, and the activation of T cells, is constitutively expressed on the surface of CD4+CD25+ Treg cells independently of their activation status. In contrast, Nrp1 expression is down-regulated in naive CD4+CD25– T cells after TCR stimulation. Furthermore, CD4+Nrp1high T cells express high levels of Foxp3 and suppress CD4+CD25– T cells. Thus, Nrp1 constitutes a useful surface marker to distinguish Treg cells from both naive and recently activated CD4+CD25+ non-regulatory T cells.