Activated neutrophils increase sickle red blood cell (SRBC) retention/adhesion in the pulmonary circulation. This study investigates the role of neutrophil activation and SRBC retention/adhesion in the pulmonary circulation through a mechanism that involves increasing phosphatidylserine (PS) exposure on the external membrane surface of the SRBC (PS-exposed). Using flow cytometry, double labeling studies were performed with a calcium-dependent phospholipid binding protein, annexin V-fluorescein isothiocyanate fluorescence and the erythroid-specific marker, glycophorin A to assess for the percent of PS-exposed normal and SRBCs at baseline and following co-incubation with activated neutrophils (AN). Additional studies were performed which assessed retention/adhesion of SRBCs in the isolated rat lung using ⁵¹Cr-labeled SRBC alone, SRBC+AN, SRBC+AN+zileuton, and SRBC+AN+ annexin V. Specific activities of lung and perfusate were measured and the number of retained SRBCs/gram lung was calculated. Flow cytometry demonstrated that ANs increased the percent of PS-exposed normal and SRBCs. The 5-lipoxygenase inhibitor, zileuton, attenuated AN-mediated increases in PS-exposed SRBCs and decreased SRBC retention/adherence in the lung on histologic sections. Similarly, in the isolated perfused lung and in histologic lung sections, retention/adherence of SRBCs cloaked with annexin V was attenuated in the presence of ANs. We conclude that ANs enhance the adhesion of SRBCs to vascular endothelium by increasing red blood cell membrane externalization of PS. Zileuton attenuation of AN-mediated SRBC PS externalization suggests that a 5-lipoxygenase product(s), secreted by the AN, plays a vital role in altering the adhesive properties of PS-exposed SRBCs to vascular endothelium.