The possible adjuvant effect of diesel exhaust particles (DEP) on the response to the model allergen ovalbumin (OA) was studied in BALB/c mice using the popliteal lymph node (PLN) assay. In addition to changes in PLN weight, cell numbers and cell proliferation, specific serum IgE anti-OA antibody levels were measured. OA inoculated together with DEP into one hind footpad gave a significantly augmented response (increase in weight, cell numbers and cell proliferation) in the draining popliteal lymph node as compared to DEP or OA alone. Also, the local lymph node response was of longer duration when DEP were given with the allergen. Experiments in thymus-deficient nu/nu mice indicated that the lymph node response observed in BALB/c mice was of a specific immunologic character and not an unspecific inflammatory reaction. The OA-specific IgE response was increased in mice receiving OA together with DEP as compared to the response in mice receiving OA without DEP. Carbon black (CB) was given with and without OA in some experiments, as a surrogate for the non-extractable core of DEP. CB was found to resemble DEP in its capacity to increase the local lymph node response and serum specific IgE response to OA, but CB appeared to be slightly less potent than DEP. Thus, both DEP and CB had a significant adjuvant effect on the local immune-mediated inflammatory response and on the systemic specific IgE response to allergen. The results indicate that the non-extractable particle core contributes substantially to the adjuvant activity of DEP.