Peroxisome proliferator-activated receptor (PPAR) agonists, fibrates and thiazolidinediones, are commonly used drugs in the treatment of dyslipidemia and diabetes. Their targets, PPARα and PPARγ, have also been shown to have a role in the regulation of inflammatory responses linking metabolism and inflammation. In the present study we investigated the effects of PPAR agonists on macrophage activation. In addition to the proinflammatory classical activation, we also focused on interleukin (IL) 4 and 13 -induced alternative activation which is a significant macrophage phenotype in tissue repairing processes and in fibrosing diseases. PPARα agonists GW7647 and fenofibrate as well as PPARγ agonist GW1929 inhibited lipopolysaccharide-induced classical macrophage activation and production of the characteristic biomarkers of this phenotype, i.e. IL-6 and nitric oxide, in murine J774 macrophages. Remarkably, the PPARα agonists also inhibited IL-4 and IL-13 –induced expression of alternative activation markers arginase-1, fizz1 and mannose receptor 1 whereas the PPARγ agonist GW1929 enhanced their expression in J774 macrophages. The PPARα agonists GW7647 and fenofibrate also attenuated the production of alternative activation markers chemokine (C-C motif) ligand 13 and plateletderived growth factor in human THP-1 macrophages. The present findings show that PPARα and PPARγ agonists differently regulate classical and alternative macrophage phenotypes. Furthermore, PPARα activation was introduced as a novel concept to down-regulate alternative macrophage activation indicating that PPARα agonists have therapeutic potential in conditions associated with aberrant alternative macrophage activation such as fibrosing diseases.