IL‐12(p70), a heterodimer composed of two subunits (p35 and p40), is a key cytokine for Th1 mediated inflammatory responses. We dissected the role of IL‐12 in the development of 2,4,6‐trinitrobenzene sulfonic acid (TNBS)‐induced colitis by studying mice deficient in IL‐12p40, IL‐12p35, or IL‐12Rβ1. TNBS‐treated IL‐12Rβ1^–/– and IL‐12p35^–/– mice developed only a mild disease associated with low level IL‐18 expression in IL‐12p35^–/– mice. In contrast, IL‐12p40^–/– mice developed more severe colitis than wild‐type mice associated with high level colonic IL‐18 expression. Administration of IL‐12p40 neutralizing mononuclear antibody dramatically increased pathology in IL‐12p35^–/– mice similar to disease scored in IL‐12p40^–/– mice. Numbers of IFN‐γ‐producing cells infiltrating the lamina propria were comparably augmented in the different groups of IL‐12‐mutant and wild‐type mice. These results demonstrate that IL‐12p40, in contrast to IL‐12p70, inhibits TNBS‐induced colitis and IL‐18 expression independent of IFN‐γ.