Interleukin-10 (IL-10) is a cytokine that has been tested in different clinical trials based on its ability to down regulate T helper 1-type responses, namely IFN-gamma secretion and activation of monocytes/macrophages. There is also evidence in different animal models, that IL-10 could be useful in controlling Th2-mediated inflammatory processes. However, IL-10 also displays immunostimulatory properties especially on B cells and activated CD8(+)T cells. These seemingly divergent effects may explain the apparent lack of activity or adverse effects observed after IL-10 treatment in several animal models or clinical trials. Nevertheless, the ability of IL-10 to induce the differentiation of a subset of regulatory CD4(+)T cells (Tr1) and the importance of IL-10 for the in vivo function of regulatory T cells tends to support the view of IL-10 as a crucial cytokine in the control of immune responses. In different in vivo models, these cells were shown to inhibit Th1 and Th2-type inflammatory responses through the secretion of IL-10. These Tr1 cells may thus be used in specific cellular therapy in order to deliver IL-10 precisely at the site of inflammation.