The effects of diesel exhaust particles (DEPs) on release of interleukin-1 (IL1) and tumor necrosis factor alpha (TNFa) by rat alveolar macrophages (AMs) were examined. AMs collected from male Sprague-Dawley-rats by bronchoalveolar lavage (BAL) were incubated with suspensions containing 0, 5, 10, 20, 50, or 100 micrograms per milliliter (microg/ml) DEPs, or methanol washed DEPs or methanolic DEP extracts containing equivalent DEP concentrations for 24 hours. The effects on spontaneous release of IL1 and TNFa by the AMs were evaluated using a murine thymocyte comitogen assay. The effects on release of IL1 and TNFa following priming of the AMs with 0.1microg/ml lipopolysaccharide (LPS) or 5 units per ml recombinant interferon-gamma (INFg) were also determined. DEPs at concentrations of 50 and 100microg/ml stimulated a significant spontaneous release of IL1. The DEP methanol extracts at 100microg/ml also stimulated spontaneous release of IL1. Methanol washed DEPs had no effect on spontaneous IL1 release. These findings were taken as evidence that the stimulatory effect of DEPs on AM production of IL1 was due primarily to extractable organic components. Neither DEPs, methanol washed DEPs, nor the DEP methanol extracts stimulated spontaneous release of TNFa. DEP at concentrations of 20 and 50microg/ml significantly inhibited LPS and INFg stimulated production of IL1. The same concentrations of the methanolic extracts, but not washed DEPs, also significantly inhibited production of LPS and INFg stimulated IL1. None of the DEP preparations significantly altered production of TNFa in response to LPS or INFg stimulation. The authors conclude that in- vitro DEPs or their organic extracts are capable of stimulating AMs to release IL1, but not TNFa. This suggests that IL1 may play an important role in the development of DEP induced inflammatory and immune responses.