Previous studies have shown that the chronic inflammation observed in the colon of IL-10-deficient (IL-10−/−) mice is mediated by CD4+ Th1 T cells and is dependent on the presence of IFN-γ for its initial development. As CD4+ T cells from IL-10−/− mice will cause colitis when transferred into recombinase-activating gene (Rag)-deficient recipients, we considered the possibility that the recipients’ NK cells could be an important source of IFN-γ for the development of colitis. Therefore, the ability of IL-10−/− CD4+ T cells to cause colitis in Rag-deficient recipients that had been depleted of NK cells was tested. Contrary to our expectations, NK cell-depleted recipients of IL-10−/− CD4+ T cells developed accelerated disease compared with nondepleted recipients. Furthermore, CD4+ T cells from normal mice (IL-10+/+) also caused colitis in NK cell-depleted recipient mice, but not in nondepleted recipients. NK cells inhibited effector CD4+ CD45RB high T cells, and subsequent experiments showed that this effect was dependent on perforin. Thus NK cells can play an important role in down-regulating Th1-mediated colitis by controlling the responses of effector T cells to gut bacteria.