Neutrophils activated by monocyte supernatants disturb the barrier function of a human pulmonary microvascular endothelial cell monolayer. Human pulmonary microvascular endothelial cells were grown to confluence on a Transwell filter system. FITC-labeled albumin, neutrophils, and supernatant were added to the upper chamber. Presence of FITC-albumin in the bottom chamber was read at the indicated time points. Experiments were performed with monocyte supernatants obtained after incubation with matched or unmatched HLA class II antibodies as indicated (continuous lines). Thrombin was used as a positive control. Experiments were repeated in the presence of DPI to block NADPH oxidase activity (dashed lines). P < .01 for matched versus unmatched supernatant at 30 minutes and 60 minutes in the absence, but not in the presence, of DPI; n = 5 for each experiment. Results are given as mean values ± SDs.