1. As lipopolysaccharide is a major stimulator of neutrophil responses during Gram-negative bacterial infections, we studied its effect on the membrane expression of neutral endopeptidase 24.11/CD10 on neutrophils in a model of endotoxaemia in vitro. Lipopolysaccharide added to human whole-blood induced a marked and sustained CD10/neutral endopeptidase upregulation that was already detectable at 0.1 ng/ml and was maximal at a lipopolysaccharide concentration of 10 ng/ml. 2. We observed that neither tumour necrosis factor-α nor any newly synthesized protein was involved in the upregulation observed after 1 h incubation with 10 ng/ml lipopolysaccharide. 3. We further studied whether the lipopolysaccharide-induced CD10/neutral endopeptidase upregulation was mediated by lipopolysaccharide binding to the neutrophil CD14 receptor. Incubation of whole blood with an anti-CD14 monoclonal antibody before the addition of 0.1 ng/ml or 0.5 ng/ml lipopolysaccharide resulted in complete inhibition of CD10/neutral endopeptidase upregulation. In contrast, at a lipopolysaccharide concentration of 10 ng/ml, the anti-CD14 monoclonal antibody had an incomplete blocking effect. 4. The differential requirement for the CD14 receptor, depending on the lipopolysaccharide dose, was confirmed by the study of a patient suffering from paroxysmal nocturnal haemoglobinuria (in whom a complete defect in neutrophil CD14 expression was previously documented) . 5. We finally confirmed these results using purified neutrophils, demonstrating that lipopolysaccharide-induced CD10/neutral endopeptidase upregulation depends on direct interaction with neutrophil CD14.