Peroxisome proliferator activated receptors (PPARs) are ligand-activated transcription factors with diverse actions including adipocyte differentiation and lipid metabolism. Recent studies have revealed anti-inflammatory activities, but the majority of these studies have been performed in monocyte/macrophages. In these studies, we investigate the effects of PPAR ligands in murine mitogen-activated splenocytes. Ciglitazone, a PPARγ ligand, consistently decreased IFN-γ and IL-2 production by mitogen-activated splenocytes and had modest effects on splenocyte proliferation. The effects of WY14, 643, a representative of the fibrate class of PPARα ligands, on splenocyte proliferation and IL-2 levels are less marked than those observed with the PPARγ ligand. In addition, treatment with WY14, 643 and other fibrates led to marked increases in supernatant concentrations of IL-4. However, treatment with a potent and specific PPARα ligand (GW7, 647) did not augment IL-4. Also, WY14, 643 induced IL-4 expression in splenocytes from PPARα knockout mice, suggesting that the fibrate effect on IL-4 was largely through a PPARα-independent mechanism. This increase in IL-4 was associated with and causatively related to augmented expression of CD23 by CD45R/B220+ cells. We also demonstrate that PPARγ gene expression is up-regulated in T cells by mitogen activation, that it is positively regulated by IL-4 and WY14, 643, and that it is blocked by anti-IL-4. Finally, we demonstrate that WY14, 643 can modestly augment IL-4 promoter activity in a PPARα-independent manner. In concert, these findings support the roles of PPAR ligands in modulating inflammatory responses involving lymphocytes but also establish potent effects of the fibrate class of PPARα ligands on IL-4 expression that are receptor independent.