The authors investigated the effect of diesel exhaust (DE) on cytokine expression in murine lung tissues. BALB/c mice were exposed to DE for 1 month at different dose levels of DE (low dose: diesel exhaust particles [DEP] 100 micro g/m(3); high dose: 3mg/m(3)). After exposure, the authors examined mRNA expression of cytokines (tumor nocrosis factor alpha [TFN-alpha], Interleukin [IL]-1beta, IL-4, IL-6, IL-10, IL-12p40, and interferon gamma [IFN-gamma] and inducible nitric oxide synthase (iNOS) in the lung, and also measured the secretion of TNF-alpha and IL-10 protein by alveolar macrophages (AM). The mRNA expression levels of inflammatory cytokines (TNF-alpha, IL-1beta, IL-6, IL-12p40, IFN-gamma) and iNOS, which are important for host defense, were suppressed significantly. However, the IL-10 mRNA level was increased by DE exposure. The IL-4 mRNA level was increased by low-dose DE exposure but suppressed by high-dose DE exposure. TNF-alpha and IL-10 secretion by AM paralleled mRNA expression. Chronic inhalation of DE affects cytokine expression in murine lung. These results suggest that DE alters immunological responses in the lung and may increase susceptibility to pathogens, and that increased IL-4 expression by low-dose DE exposure may induce allergic reaction such as asthma.