Macrophages incubated with OVA in the presence of TGF-β 2 induce immune deviation in vivo (impaired delayed hypersensitivity and IgG2a Ab production) when injected into naive, syngeneic mice. OVA-specific TCR transgenic naive T cells (DO11. 10 T cells) produce Th1-type cytokines when stimulated in vitro with OVA-pulsed peritoneal exudate cells (PEC), but if PEC are first treated with TGF-β 2 and then pulsed with OVA, the T cells secrete Th2-type cytokines instead. In this study, we investigated the mechanisms that are involved in the modified Ag-presenting functions of macrophages by TGF-β 2 pretreatment. We have found that: 1) TGF-β 2 impaired the capacity of PEC to produce IL-12 and to express CD40; 2) reduced CD40 expression on TGF-β 2-treated PEC impaired IL-12 production when the cells were cocultured with DO11. 10 T cells; 3) the failure of TGF-β 2-treated PEC to stimulate DO11. 10 T cells to secrete IFN-γ was due to their impaired IL-12 production. From these results, we conclude that TGF-β 2 treatment impairs the ability of macrophages to produce IL-12 and to express CD40. As a consequence, TGF-β 2-treated PEC fail to promote development of pT cells toward the Th1 phenotype.