Trans-10, cis-12-conjugated linoleic acid (t10c12-CLA) has been shown to alter immune function. PPARgamma has been shown to potentially play an important role in regulating inflammatory and immune responses by modulating the activity of monocytes and macrophages. Previous studies have indicated that the phagocytic capacity of porcine peripheral blood polymorphonuclear cells (PMN) was enhanced by the culture supernatant fraction from t10c12-CL-stimulated porcine peripheral blood mononuclear cells (PBMC) but not by t10c12-CLA itself. In the present study, we examined the effects of t10c12-CLA on PPARgamma and TNF-alpha expression of porcine PBMC and the phagocytic capacity of PMN. t10c12-CLA increased TNF-alpha mRNA expression and production by PBMC. The phagocytic capacity of porcine PMN was enhanced by either culture supernatant fraction from PBMC treated with t10c12-CLA or recombinant porcine (rp) TNF-alpha. Anti-rpTNF-alpha polyclonal antibody inhibited the enhancement of PMN phagocytic capacity. t10c12-CLA also up regulated PPARgamma mRNA expression in porcine PBMC. Bisphenol A diglycidyl ether, a PPARgamma antagonist, not only completely negated the t10c12-CLA-stimulating effects on TNF-alpha expression and production by porcine PBMC, but also decreased the enhancement of PMN phagocytic capacity by the t10c12-CLA-stimulated porcine PBMC culture supernatant fraction. These results suggest that t10c12-CLA has an immunostimulating effect on porcine PMN phagocytic capacity, which is mediated by TNF-alpha from PBMC via a PPARgamma-dependent pathway.