Progressing tumors in man and mouse are often infiltrated by dendritic cells (DCs). Deficient antitumor immunity could be related to a lack of tumor-associated antigen (TAA) presentation by tumor-infiltrating DCs (TIDCs) or to a functional defect of TIDCs. Here we investigated the phenotype and function of TIDCs in transplantable and transgenic mouse tumor models. Although TIDCs could encompass various known DC subsets, most had an immature phenotype. We observed that TIDCs were able to present TAA in the context of major histocompatibility complex class I but that they were refractory to stimulation with the combination of lipopolysaccharide, interferon γ, and anti-CD40 antibody. We could revert TIDC paralysis, however, by in vitro or in vivo stimulation with the combination of a CpG immunostimulatory sequence and an anti-interleukin 10 receptor (IL-10R) antibody. CpG or anti–IL-10R alone were inactive in TIDCs, whereas CpG triggered activation in normal DCs. In particular, CpG plus anti–IL-10R enhanced the TAA-specific immune response and triggered de novo IL-12 production. Subsequently, CpG plus anti–IL-10R treatment showed robust antitumor therapeutic activity exceeding by far that of CpG alone, and elicited antitumor immune memory.