Airborne endotoxins play a role in a variety of occupational diseases such as byssinosis and humidifier's disease and are associated with pulmonary and systemic symptoms. An excess generation of oxygen free radicals (including superoxide anion, O2-) by inflammatory cells has been suggested in endotoxemia. We have studied the release of superoxide from guinea-pig lung lavage and interstitial cells and blood monocytes (BMs) at different times after an acute inhalation exposure to bacterial endotoxin. O2- generation was measured by the O2- dismutase-inhibitable reduction of ferricytochrome c, after stimulation with phorbol-myristate-acetate (PMA) or opsonized zymosan (OZ). After endotoxin exposure, the spontaneous release of O2- remained unchanged for the three cell types. From 4 h after exposure until 48 h afterwards, lung lavage cells produced more O2- after PMA or OZ stimulation than did cells from unexposed guinea pigs. The pattern of O2- generation by interstitial cells followed that of lung lavage macrophages. O2- production remained unchanged in BMs at all times. These results suggest that endotoxin inhalation induced a priming of lavage and interstitial cells, mainly macrophages, associated with an increase in O2- production. As O2- production by BMs remained unchanged, this cell is unlikely to be responsible for the systemic symptoms seen after endotoxin inhalation.