Exposure to endotoxin and to its purified derivative lipopolysaccharide (LPS) is related to several occupational pulmonary diseases and to severe domestic asthma. An inhalation of a given dose of pure LPS produces both a systemic and a bronchial inflammatory response. Information on the dose-response relationship to inhaled LPS in normal subjects is a prerequisite to define the safety threshold of exposure. In the present study, the clinical and inflammatory responses to rising doses of inhaled LPS was evaluated. Nine normal volunteers were challenged weekly by inhalation with saline, 0.5, 5, and 50 microg LPS (Escherichia coli). The response determinators are the clinical symptoms, fever, FEV1, blood polymorphonuclear neutrophils (PMNs) with their level of activation (measured by luminol enhanced-chemiluminescence), and both the blood and the urine concentrations of the C-reactive protein (CRP). To assess the bronchial inflammatory response, an induced sputum was obtained 6 h after each dose of LPS, and the total and differential cell counts as well as the MPO, ECP, and TNF-alpha concentrations were measured. Compared with the saline, an inhalation of 0.5 microg LPS induces a significant decrease in the PMN luminol-enhanced chemiluminescence (p < 0.01), which could reflect a process of margination and/or extravascular sequestration of activated PMN. Inhalation of 5 microg LPS is associated with a significant rise in blood CRP (p < 0.01) and PMNs (p < 0.001) and in sputum PMNs (p < 0.05), monocytes (p < 0.05), and MPO (p < 0.05). Inhalation of 50 microg LPS was characterized by a significant increase in temperature (p < 0.01), blood PMNs (p < 0.001), blood and urine CRP (p < 0.01 and < 0.01), and sputum PMNs (p < 0.001), monocytes (p < 0.05), lymphocytes (p < 0.05), MPO (p < 0.01), TNF-alpha (p < 0.01), and ECP (p < 0.01) while five subjects develop symptoms. In normal subjects, the response to inhaled LPS is dose-related, the most sensitive markers of LPS-induced inflammation being the blood PMNs count with their level of activation, the blood CRP concentration, and the sputum PMNs count. The no-response threshold to an acute inhalation of LPS is less than 0.5 microg.