IL-10-deficient (IL-10−/−) mice develop chronic enterocolitis mediated by CD4+ Th1 cells producing IFN-γ. Because IL-12 can promote Th1 development and IFN-γ production, the ability of neutralizing anti-IL-12 mAb to modulate colitis in IL-10−/− mice was investigated. Anti-IL-12 mAb treatment completely prevented disease development in young IL-10−/− mice. Treatment of adult mice resulted in significant amelioration of established disease accompanied by reduced numbers of mesenteric lymph node and colonic CD4+ T cells and of mesenteric lymph node T cells spontaneously producing IFN-γ. In contrast, anti-IFN-γ mAb had minimal effect on disease reversal, despite a significant preventative effect in young mice. These findings suggested that IL-12 sustains colitis by supporting the expansion of differentiated Th1 cells that mediate disease independently of their IFN-γ production. This conclusion was supported by the finding that anti-IL-12 mAb greatly diminished the ability of a limited number of CD4+ T cells expressing high levels of CD45RB from diseased IL-10−/− mice to expand and cause colitis in recombination-activating gene-2−/− recipients, while anti-IFN-γ mAb had no effect. Furthermore, IL-12 could support pathogenic IL-10−/− T cells stimulated in vitro in the absence of IL-2. While these studies show that IL-12 plays an important role in sustaining activated Th1 cells during the chronic phase of disease, the inability of anti-IL-12 mAb to abolish established colitis or completely prevent disease transfer by Th1 cells suggests that additional factors contribute to disease maintenance.