Lymphoid organs contain a B220(+)CD11c(+)NK1.1(+) cell population that was recently characterized as a novel dendritic cell (DC) subset that functionally overlaps with natural killer (NK) cells and plasmacytoid DCs (PDCs). Using Siglec-H and NK1.1 markers, we unambiguously dissected B220(+)CD11c(+) cells and found that PDCs are the only professional interferon (IFN)-alpha-producing cells within this heterogeneous population. In contrast, B220(+)CD11c(+)NK1.1(+) cells are a discrete NK cell subset capable of producing higher levels of IFN-gamma than conventional NK cells. Unlike DCs, only a minute fraction of B220(+)CD11c(+)NK1.1(+) cells in the spleen expressed major histocompatibility complex class II ex vivo or after stimulation with CpG. Consistent with being a NK cell subset, B220(+)CD11c(+)NK1.1(+) cells depended primarily on interleukin 15 and common cytokine receptor gamma chain signaling for their development. In terms of function, expression of distinctive cell surface receptors, and location in lymphoid organs, NK1.1(+)B220(+)CD11c(+) appear to be the murine equivalent of human CD56(bright) NK cells.