Natural interferon (IFN)-producing cells (IPCs) recognize certain viruses and DNA containing deoxycytidylate-phosphate-deoxyguanylate (CpG) motifs through the toll-like receptor (TLR) 9, resulting in secretion of IFN-α, interleukin 12 (IL-12), and proinflammatory chemokines. Human IPCs are found mainly in inflamed lymph nodes, where they are presumably recruited from the blood to activate both innate and adaptive responses to microbial infections. Demonstrating IPC recruitment and function in murine infection models has been difficult because multiple antibodies are required to distinguish IPCs from other immune cells and very few IPCs can be recovered from lymph nodes. Here we describe a monoclonal antibody (mAb) that exclusively detects murine IPCs in all lymphoid organs under both normal and Inflammatory conditions. Using this antibody, we demonstrate that IPCs are normally present in the T-cell zone of lymph nodes and spleen and that inoculation of peripheral tissues with inflammatory stimuli triggers recruitment of IPC into sentinel lymph nodes, whether the stimuli are able to directly stimulate IPCs through TLR or not. Remarkably, we show that incubation of IPCs with the antibody in vitro or administration of the antibody in vivo dramatically reduce secretion of IFN-α in response to CpG DNA without causing IPC depletion. Thus, the antibody identifies an IPC-specific surface molecule that, when engaged, inhibits IFN-α secretion.