Mo (p40) 2 is a potent IL-12 antagonist that interacts strongly with the beta 1 subunit of the IL-12R to block binding of moIL-12 to the high-affinity mouse IL-12R. Mo (p40) 2, alone or in synergy with the 2B5 mAb specific for the moIL-12 heterodimer, blocked IL-12-induced responses in vitro, Mo (p40) 2 was thus used alone or with 2B5 mAb to examine the role of IL-12 in vivo, Mo (p40) 2 caused a dose-dependent inhibition of both the rise in serum IFN-gamma levels in mice injected with endotoxin and the Th1-like response to immunization with KLH. Treatment with mo (p40) 2 plus 2B5 anti-moIL-12 mAb also suppressed DTH responses to methylated bovine serum albumin but not specific allogeneic CTL responses in vivo. In each of these models, responses seen in mice treated with mo (p40) 2+/-2B5 anti-moIL-12 mAb were similar to those observed in IL-12 knockout mice. Thus, mo (p40) 2 can act as a potent IL-12 antagonist in vivo, as well as in vitro, and is currently being used to investigate the role of IL-12 in the pathogenesis of some Th1-associated autoimmune disorders in mice.