Natural killer (NK) cells are the cells critical for inhibition of repopulation of allogenic bone marrow cells. However, it is not well known if NK cells affect autologous lymphopoiesis. Here, we observed that NK cells could inhibit pre-B cell proliferation in vitro driven by interleukin (IL)-7 in a manner dependent on IL-15. Interestingly, the great majority of expanding NK cells were Mac-1(+)B220(+), a recently identified potent interferon (IFN)-gamma producer. Indeed, IFN-gamma was produced in those cultures, and pre-B cells lacking IFN-gamma receptors, but not those lacking type I IFN receptors, were resistant to such an inhibition. Furthermore, even NK cells from mice lacking beta2-microglobulin, which were known to be functionally dampened, inhibited pre-B cell proliferation as well. Thus, activated NK cells, which were expanded selectively by IL-15, could potentially regulate B lymphopoiesis through IFN-gamma beyond the selection imposed upon self-recognition.