In order to avoid autoimmunity and excessive tissue destruction, the action of certain immunoinhibitory substances are very important for negative regulation of the immune system. Interleukin-10 (IL-10) is an important immunoregulatory cytokine which is thought to negatively affect both T cells and antigen-presenting cells in vivo. Adoptive transfer of IL-10-treated bone-marrow-derived dendritic cells (BMDCs) may be one therapeutic avenue to inhibit autoimmunity. In this study we present a comprehensive analysis of the effects of IL-10 on murine BMDC. We demonstrate that IL-10 can prevent BMDC maturation, as measured by both cytokine production and T-cell priming capacity in vitro. Furthermore, we show that IL-10 can inhibit DC maturation induced by strong stimulatory signals such as lipopolysaccharide or a mixture of cytokines (interferon-γ, tumour necrosis factor-α, IL-4). Interestingly, maturation of both T helper 1- and T helper 2-inducing DCs, characterized by the induction of high levels of interferon-γ and IL-4-production by responding T cells, respectively, was inhibited by IL-10 in vitro. Finally, our data suggest that both endogenous and exogenous IL-10 affect the T-cell stimulatory capacity of BMDCs after injection of in vitro-treated BMDCs into naïve mice. These data both support existing data as well as point towards a new understanding of the many aspects of IL-10-mediated immunosuppression.