Monocytes/macrophages (Mφ) play a pivotal role in the persistence of chronic inflammation and local tissue destruction in diseases such as rheumatoid arthritis and atherosclerosis. The production by Mφ of cytokines, chemokines, metalloproteinases and their inhibitors is an essential component in this process, which is tightly regulated by multiple factors. The peroxisome proliferator-activated receptors (PPARs) were shown to be involved in modulating inflammation. PPARγ is activated by a wide variety of ligands such as fatty acids, the anti-diabetic thiazolidinediones (TZDs), and also by certain prostaglandins of which 15-deoxy-Δ^12,14-PGJ2 (PGJ2). High concentrations of PPARγ ligands were shown to have anti-inflammatory activities by inhibiting the secretion of interleukin-1 (IL-1), interleukin-6 (IL-6) and tumour necrosis factor α (TNFα) by stimulated monocytes. The aim of this study was to determine whether PGJ2 and TZDs would also exert an immunomodulatory action through the up-regulation of anti-inflammatory cytokines such as the IL-1 receptor antagonist (IL-1Ra). THP-1 monocytic cells were stimulated with PMA, thereby enhancing the secretion of IL-1, IL-6, TNFα, IL-1Ra and metalloproteinases. Addition of PGJ2 had an inhibitory effect on IL-1, IL-6 and TNFα secretion, while increasing IL-1Ra production. In contrast, the bona fide PPARγ ligands (TZDs; rosiglitazone, pioglitazone and troglitazone) barely inhibited proinflammatory cytokines, but strongly enhanced the production of IL-1Ra from PMA-stimulated THP-1 cells. Unstimulated cells did not respond to TZDs in terms of IL-1Ra production, suggesting that in order to be effective, PPAR ligands depend on PMA signalling. Basal levels of PPARγ are barely detectable in unstimulated THP-1 cells, while stimulation with PMA up-regulates its expression, suggesting that higher levels of PPARγ expression are necessary for receptor ligand effects to occur. In conclusion, we demonstrate for the first time that TZDs may exert an anti-inflammatory activity by inducing the production of the IL-1Ra.