To investigate the effects of diesel exhaust particles (DEP) and mycobacterial infection on macrophages, we examined protein and mRNA expression levels of various cytokines, including tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, IL-12, and IL-18 in BALB/c mouse alveolar macrophages (AM) and a macrophage cell line (RAW264.7) after in vitro stimulation with diesel exhaust (DE), with and without Mycobacterium bovis bacillus Calmette-Guérin (BCG). The cells were exposed to DE freshly generated in an in vitro system. When AM were exposed to DE (mean DEP exposure level, 300 microg/m3), the levels of TNF-alpha and IL-12 decreased significantly (by 51% to 61% for TNF-alpha and by 69% to 78% for IL-12), whereas those of IL-1beta and IL-18 remained unchanged. When AM were exposed to DE and then treated with BCG, the level of TNF-alpha decreased by 45 % to 71%, whereas the level of IL-1beta increased by 154%, compared with AM treated with BCG alone. Similarly, RAW2264.7 cells were stimulated with DE with and without M. bovis BCG and cytokine mRNA levels examined by reverse transcriptase-polymerase chain reaction (R7-PCR). Longer exposure to DE decreased TNF-alpha and IL-12 mRNA levels in the RAW264.7 cells. When the cells were exposed to DE and subsequently treated with BCG, levels of TNF-alpha, IL-1beta, and IL12 mRNAs decreased compared with those of cells treated with BCG alone. These results show that DE exposure has complex and diverse effects on cytokine production by AM, and that longer exposure (>8 hours) may suppress cytokine production by AM in vitro. Longer exposure of DE may therefore suppress the host defense in the lung and may increase susceptibility to lung infections such as mycobacterial infection.