Intravascular LPS rapidly induces neutrophil sequestration in pulmonary capillaries by mechanisms that, although currently unknown, must take into account the size difference between the neutrophil and capillary diameter. To determine whether LPS alters neutrophil stiffness, and hence the ability of neutrophils to traverse capillaries, neutrophil passage through pulmonary capillaries was modeled by passage through filters with 6.5-microns pores. LPS increased retention in the pores in a concentration-dependent fashion that required the presence of heat-inactivated platelet-poor plasma, and was evident as early as 10 min after stimulation. The effect of LPS on the structural properties of the neutrophil was then studied. LPS induced f-actin reorganization in neutrophils in the presence of plasma. Disruption of actin organization and assembly with cytochalasin D completely inhibited early LPS-induced retention and attenuated retention at later timepoints, indicating that LPS-stimulated retention depends on filament organization. LPS-induced actin assembly and retention were abrogated by an antibody directed against CD14, a putative LPS receptor. CD18-dependent adherence of neutrophils contributed significantly to retention only at later timepoints with no significant contribution to retention at 20 min as determined by inhibition of adherence with the mAb 60.3. Morphometric assessment of neutrophil accumulation in the lungs of rabbits given 1 microgram LPS showed a marked increase in apparent neutrophil number, which was unaltered by antibodies to CD18, suggesting that mechanisms other than adhesion may account for accumulation in vivo. Direct measurements showed that neutrophil stiffness increased with exposure to LPS in a fashion similar to LPS-induced retention and actin organization. Pretreatment of neutrophils with cytochalasin D attenuated the increased stiffness. These data suggest that reorganization of filamentous-actin induced by LPS leads to cell stiffening and retention in capillary-sized pores. Although the organization of f-actin continues to be important in retention at later time points, adherence of cells also contributes significantly to cell retention. The changes in mechanical properties of the neutrophil may be important in the sequestration of neutrophils in pulmonary capillaries noted in endotoxemia.