CD4⁺CD25⁺ regulatory T cells (Treg) exert suppressive functions on effector T cells in vitro and in vivo. However, the exact cellular events that mediate this inhibitory action remain largely unclear. To elucidate these events, we used intravital microscopy in a model of contact hypersensitivity (CHS) and visualized the leukocyte‐endothelium interaction at the site of antigen challenge in awake C57BL/6 mice. Injection of Treg i.v. into sensitized mice at the time of local hapten challenge significantly inhibited rolling and adhesion of endogenous leukocytes to the endothelium. A similar inhibition of leukocyte recruitment could be recorded after injection of Treg‐derived tissue culture supernatant. Thus, these data indicate that soluble factors may account for the suppressive effects. Accordingly we found that IL‐10, but not TGF‐β, was produced by Treg upon stimulation and that addition of anti‐IL‐10 antibodies abrogated the suppressive effects of Treg and tissue culture supernatant in CHS reactions. Moreover, CD4⁺CD25⁺ T cells isolated from IL‐10^–/– mice were not able to suppress the immune response induced by hapten treatment in C57BL/6 mice. In conclusion, our data suggest that cytokine‐dependent rather than cell‐cell contact‐dependent mechanisms play a pivotal role in the suppression of CHS reactions by Treg in vivo.