The cytokine IL-12 plays a critical role in inducing the production of IFN-gamma from T and NK cells and in the polarization of T cells towards the Th1 phenotype. IL-12 is comprised of two subunits (IL-12p40 and IL-12p35) that together form the biologically active p70 molecule, and IL-12 functions via binding to a heterodimeric receptor (IL-12Rbeta1 and IL-12Rbeta2). Previous studies utilizing mice deficient for either the IL-12 cytokine or the IL-12-induced signaling molecule STAT4 have established a critical role for IL-12 during infection with Leishmania major. However, these studies warrant careful re-interpretation in light of the recent discovery of the IL-12-related cytokine, IL-23, which utilizes the IL-12p40 chain in combination with an IL-12p35-related molecule, called p19, and a receptor comprised of the IL-12Rbeta1 chain plus a unique chain referred to as IL-23R. We analyzed the course of L. major infection in mice deficient for the IL-12-specific IL-12Rbeta2 subunit in order to assess the role of IL-12 signaling without disruption of the IL-23 pathway. After infection with L. major, IL-12Rbeta2KO mice of a resistant background (C57Bl/6) developed large cutaneous lesions similar to those developed by susceptible BALB/c mice. Draining lymph node cells from L. major-infected IL-12Rbeta2KO mice released the Th2 cytokines IL-4 and IL-5 after in vitro stimulation with Leishmania lysate but were completely devoid of IFN-gamma, consistent with a default towards a strong parasite-specific Th2 response. L. major-infected IL-12Rbeta2KO mice were also devoid of parasite-specific IgG2a antibodies, and interestingly, their footpad lesions ulcerated earlier than those of susceptible BALB/c mice.