Extracellular nucleotides activate ligand-gated P2XR ion channels and G protein-coupled P2YRs. In this study we report that intradermal administration of ATPγS, a hydrolysis-resistant P2 agonist, results in an enhanced contact hypersensitivity response in mice. Furthermore, ATPγS enhanced the induction of delayed-type hypersensitivity to a model tumor vaccine in mice and enhanced the Ag-presenting function of Langerhans cells (LCs) in vitro. Exposure of a LC-like cell line to ATPγS in the presence of LPS and GM-CSF augmented the induction of IA, CD80, CD86, IL-1β, and IL-12 p40 while inhibiting the expression of IL-10, suggesting that the immunostimulatory activities of purinergic agonists in the skin are mediated at least in part by P2Rs on APCs. In this regard, an LC-like cell line was found to express mRNA for P2X 1, P2X 7, P2Y 1, P2Y 2, P2Y 4, P2Y 9, and P2Y 11 receptors. We suggest that ATP, when released after trauma or infection, may act as an endogenous adjuvant to enhance the immune response, and that P2 agonists may augment the efficacy of vaccines.