Previous studies in BALB/c mice revealed an adjuvant effect of di-(2-ethylhexyl) phthalate (DEHP) to simultaneously administered ovalbumin. DEHP is the most commonly used phthalate plasticizer. In vivo formed metabolites of DEHP are peroxisome-proliferator-activated receptor (PPAR) ligands, a group of chemicals that may have immunomodulatory properties. To study whether the PPARalpha receptor was involved in the adjuvant effect of DEHP, PPARalpha-deficient 129/Sv mice were exposed intraperitoneally to a mixture of OVA and DEHP, and the OVA-specific IgE, IgG1 and IgG2a responses were compared to the corresponding responses in the wild-type strain. The study showed that the adjuvant mechanism of DEHP is mediated through a PPARalpha-independent mechanism. Compared to mice only given OVA, DEHP induced highly increased levels of OVA-specific IgG1 and IgG2a, both in the wild-type and in the PPARalpha knock-out strains, indicating that DEHP is a mixed Th1/Th2 adjuvant.