Patients with cystic fibrosis (CF) and chronic Pseudomonas aeruginosa lung infection have a very high load of endotoxins in their lungs. However, sepsis practically never occurs in this group of patients and the presence of tumor necrosis factor (TNF)-alpha (one of the mediators of septic shock) in serum from chronically infected CF patients is contentious. The purpose of this study was to investigate the effect of hyperimmune serum from patients with CF on lipopolysaccharide (LPS, endotoxin)-induced TNF secretion from human peripheral blood mononuclear cells (PBMC). PBMC were purified from healthy donors and stimulated with a mixture of purified LPS from P. aeruginosa and serum from chronically infected CF patients or healthy controls. TNF in the cell supernatants was detected by an enzyme-linked immunosorbent assay method. CF sera showed a pronounced potentiating effect on TNF secretion from human PBMC induced by LPS from P. aeruginosa. In comparison, serum from healthy controls did not have this effect. By contrast, CF serum and serum from healthy controls showed only little potentiating effect when using LPS from Salmonella abortus equi at concentrations above 0.01 microgram/ml per 2 x 10(6) PBMC. This indicates a specific interaction between P. aeruginosa LPS and CF serum which enhances TNF secretion. The TNF responses varied depending on the sera used in the preincubation with LPS, and correlated positively to the specific IgG, IgA, and IgM anti-P. aeruginosa LPS titers of the sera. However, since TNF is hardly detectable in sera from these patients another LPS- and/or TNF-inhibitory activity may be present in these sera.