Active suppression mediated by CD4+ CD25+ T regulatory (Tr) cells plays an important role in the down-regulation of T cell responses to both foreign and self-Ags. Platelet factor 4 (PF4), a platelet-derived CXC chemokine, has been shown to strongly inhibit T cell proliferation as well as IFN-γ and IL-2 release by isolated T cells. In this report we show that human PF4 stimulates proliferation of the naturally anergic human CD4+ CD25+ Tr cells while inhibiting proliferation of CD4+ CD25− T cells. In coculture experiments we found that CD4+ CD25+ Tr cells exposed to PF4 lose the ability to inhibit the proliferative response of CD4+ CD25− T cells. Our findings suggest that human PF4, by inducing Tr cell proliferation while impairing Tr cell function, may play a previously unrecognized role in the regulation of human immune responses. Because platelets are the sole source of PF4 in the circulation, these findings may be relevant to the pathogenesis of certain immune-mediated disorders associated with platelet activation, such as heparin-induced thrombocytopenia and autoimmune thrombocytopenic purpura.