The adjuvant effect of DEP on ovalbumin (OVA) sensitization and its underlying mechanism were studied. Brown Norway rats were exposed to saline or DEP (5 mg kg) on day 1 followed by exposure to OVA (40 mg m3: 30 min) or saline on day 2.8. 15. and 29. Animals were sacrificed and analyzed for inflammatory markers in the lavage fluid, cysteine and glutathione (GSH) levels in alveolar macrophages (AM) and lymphocytes. serum IgG and IgE. and IFN-y and IL-4 mRNA levels (standardized with G3PDH) in lung tissue. OVA or DEP exposure alone did not result in abnormal levels of inflammatory cells, LDH, or total protein in the lavage fluid. In the combined exposure, however, these markers were significantly increased comparing to either OVA- or DEP-exposed group. The adjuvent effect of DEP on OVA sensitization was evidenced by marked increases in serum OVA-specific IgG (5.7-fold) and IgE (3-fold) levels, and an increase in IL-4 but a decrease in IFN-y mRNA levels in lung tissue. DEP exposure had little effect on the cellular level of cysteine or GSH in AM or lymphocytes. The OVA exposure, however, markedly reduced GSH levels in both cell types. In combined exposure, the level of GSH in lymphocytes was further decreased, thus indicating an interactive effect between DEP and OVA exposures. These results support the concept that depletion of intracellular GSH in lymphocytes hunts the response to a TH2 type may play a key role in DEP-augmented pulmonary allergic responses.