The capacity of mouse spleen conventional dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs) to produce interferon-γ (IFN-γ) or IFN-α was assessed, and compared with that of natural killer (NK) cells and the recently identified interferon-producing killer dendritic cells (IKDCs), both of which are frequent contaminants in DC preparations. Fully developed cDCs or pDCs, if free of NK cells or IKDCs, showed little capacity for IFN-γ production. However, an early developmental form of the CD4⁻8⁺ cDC subtype, and the Ly6C⁻ Ly49Q⁻ pDC subtype, both were able to produce moderate amounts of IFN-γ, although less than IKDCs. In response to toll-like receptor 9 stimuli, both the Ly6C⁺ Ly49Q⁺ and the Ly6C⁻ Ly49Q⁻ pDC subtypes were effective producers of IFN-α. However, IKDCs, which efficiently produced IFN-γ and showed immediate cytotoxicity on NK target cells, did not produce IFN-α un-der these conditions.