['Bactericidal/permeability-increasing protein (BPI) binds lipopolysaccharide and neutralizes its toxic effects in vitro and in endotoxemic animals. Our recent work identified physiologically significant interactions between BPI, lipopolysaccharide, and mononuclear cells.', 'To determine whether the interaction between BPI and mononuclear cells is receptor mediated.', 'Labeled BPI was incubated with THP-1 cells in the presence of up to 100-fold excess of unlabeled BPI. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting were performed to evaluate competitive binding and total uptake of BPI. Crosslinking was performed to determine whether BPI binds to a single protein entity. Acid washing experiments and flow cytometric analysis were performed to determine whether BPI remains on the cellular surface. Finally, flow cytometry analysis was used to determine whether BPI incubation with THP-1 cells affects the surface expression of the lipopolysaccharide-binding protein-lipopolysaccharide receptor CD14.', 'Labeled BPI uptake was not inhibited by the presence of 100-fold excess of unlabeled BPI at 37 degrees C or 4 degrees C in the presence of azide. Uptake was not saturable under either condition with incubation concentrations up to 10 microgram/mL. Cross-linking did not show BPI bound to a single entity. Acid washing and flow cytometry experiments disclosed rapid internalization of BPI. Finally, BPI uptake by THP-1 cells had no effect on the surface expression of CD14.'] Does mononuclear cell line THP-1 internalize bactericidal/permeability-increasing protein by a non-receptor-mediated mechanism consistent with pinocytosis?