Macrophages are phagocytic cells that play essential roles in innate immunity and lipid homeostasis. The uptake of modified lipoproteins is an important early event in the development of atherosclerosis. We analyzed the ability of modified low-density lipoprotein (LDL) (oxidized and acetylated) to alter the expression and activity of arginases (ArgI and ArgII) in macrophages. We show that ArgI expression is potently induced by both oxidized and acetylated LDL in macrophages. We further show that this effect is mediated by peroxisome proliferator-activated receptors (PPAR). ArgI expression is highly responsive to agonists for PPARγ and PPARδ but not PPARα. Moreover, the induction of ArgI by both PPAR agonists and IL-4 is blocked in macrophages from PPARγ- and PPARδ-deficient mice. Functionally, PPAR activity induces macrophage activation toward a more Th2 immune phenotype in a model of Leishmania major infection. We show that PPARγ and -δ ligands promote intracellular amastigote growth in infected macrophages, and this effect is dependent on both PPAR expression and Arg activity. Collectively, our results strongly suggest that ArgI is a key marker of the alternative program triggered by PPAR in macrophages.