We show here that mouse interferon-α (IFN-α)–producing cells (mIPCs) are a unique subset of immature antigen-presenting cells (APCs) that secrete IFN-α upon stimulation with viruses. mIPCs have a plasmacytoid morphology, can be stained with an antibody to Ly6G and Ly6C (anti-Ly6G/C) and are Ly6C⁺B220⁺CD11c^loCD4⁺; unlike other dendritic cell subsets, however, they do not express CD8α or CD11b. Although mIPCs undergo apoptosis in vitro, stimulation with viruses, IFN-α or CpG oligonucleotides enhanced their survival and T cell stimulatory activity. In vivo, mIPCs were the main producers of IFN-α in cytomegalovirus-infected mice, as depletion of Ly6G⁺/C⁺ cells abrogated IFN-α production. mIPCs produced interleukin 12 (IL-12) in response to viruses and CpG oligodeoxynucleotides, but not bacterial products. Although different pathogens can selectively engage various APC subsets for IL-12 production, IFN-α production is restricted to mIPCs' response to viral infection.