Naturally occurring CD4+CD25+FoxP3+ regulatory T cells (Treg) suppress T helper (Th) cell-mediated immune responses. The cytokines IL-2 and IL-6 are known to influence Treg function. However, their relative effects on Th cells versus Treg are not well understood. Stimulation with IL-2, and to a lesser extent, IL-6, enhanced Treg proliferation, FoxP3 and CTLA4 maintenance, and suppressive function. In contrast, when IL-2 or IL-6 were added to Treg/Th cell cocultures, suppression was inhibited. The molecule SOCS3 negatively regulates responses to IL-2 and IL-6. Interestingly, unlike Th cells, Treg were found to be deficient in SOCS3 protein expression. The significance of this finding lies in the need for Treg to rapidly respond to these cytokines to prevent unwarranted immune responses to self-antigens. Overexpression of SOCS3 in Treg decreased their proliferation, FoxP3 and CTLA-4 expression and suppressive function. Thus, up-regulation of SOCS3 expression may be a useful therapeutic approach in diseases where inhibition of Treg is desirable.