The adjuvant activity of diesel exhaust particles (DEP) on systemic IgE production to ovalbumin (OA) was studied in mice after intranasal administration. The main purpose was to elucidate which part of the particles was responsible for the effect, the carbon core and/or the adsorbed organic substances. Female Balb/cA mice were immunized with OA either alone or in combination with DEP or carbon black particles (CB), the latter used as a surrogate for the non-extractable carbon core of DEP. Controls were given DEP, CB or buffer alone. The animals were immunized four times. 1 and 2 weeks after the last immunization anti OA IgE antibody in serum was analysed by enzyme linked immunosorbent assay (ELISA). An increased response to the antigen was observed in animals receiving OA together with DEP or CB, compared with animals receiving OA alone. The increased response was seen as both increased number of responding animals and increased serum anti OA IgE antibody. For OA + DEP 37% of the animals showed a serum anti OA IgE response, whereas 22% of the OA + CB animals and 10% of the OA animals responded. In conclusion, this work shows that not only DEP, but also CB have an adjuvant activity for specific IgE production after intranasal instillation. However, the activity of DEP may be more pronounced than that of CB. The results imply that both the organic matter adsorbed to DEP and the non-extractable carbon core are responsible for the observed adjuvant effect.