Diesel exhaust particles are reported to increase the specific IgE response to ovalbumin (OVA) and pollen. Evidence has been provided that the particle core contributes to this adjuvant activity. The purpose of our study was to investigate the effect of well-defined simple particles, polystyrene particles (PSP), on the production of allergen-specific IgE in a mouse model. The IgE adjuvant effect of PSP was investigated in experiments using intranasal (i.n.) instillation, intratracheal (i.t.) instillation or intraperitoneal (i.p.) injection. Delayed and cumulative adjuvant effects were investigated by giving mice i.p. injections with PSP 1-3 days, or on 4 consecutive days before OVA, respectively. The levels of allergen-specific and total IgE were measured. Irrespectively of immunisation route and protocol, OVA in combination with PSP elicited increased levels of both allergen-specific and total IgE when compared with OVA alone. Therefore, in the experimental model, particles were found to augment the specific IgE response to an allergen even when the allergen was introduced several days after the particles. These findings imply that individuals exposed to particulate air pollution at one point of time may develop an increased reaction towards allergens inhaled later that day or even several days after the particle exposure.