Increased antigen-specific IgE expression is a hallmark of the allergic response in mice. IgG1 may also be involved. Co-injection of mice with diesel exhaust particles (DEP) and ovalbumin three times over a 2 week period lead to a rapid and marked elevation of ovalbumin-specific IgE, IgG1 and also IgG2a, compared with ovalbumin alone. When DEP were injected 1 day before or after ovalbumin on each occasion, their adjuvant effect was considerably muted, suggesting that the adjuvant effect of DEP is short-lived, or that a physical interaction between ovalbumin and DEP is required. DEP were extracted with methylene chloride. Both the resulting core carbon particles and the organic extract enhanced ovalbumin specific IgE and IgG1 levels. Thus the adjuvant effect of DEP in this model is due both to the physical and the chemical attributes of the particles. The tricyclic hydrocarbons phenanthene (the most prevalent polycyclic aromatic hydrocarbon in DEP) and anthracene were both capable of enhancing antigen-specific IgE and IgG1 production. The phenolic antioxidant, butylated hydroxyanisole, which can affect gene expression via the antioxidant responsive element (ARE), had a lesser effect. Two agonists for the aryl hydrocarbon receptor, 3-methychloranthrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin, either were without effect or suppressed the response, suggesting that DEP adjuvancy may not be mediated by this receptor.