Acute cigarette smoking increases the sequestration of neutrophils in the lungs of humans. This may be due to the delayed transit of cells in the pulmonary microcirculation, which may result from a reduction in cell deformability as suggested by in vitro studies of smoke-exposed neutrophils. In order to support this hypothesis we wished to determine if a reduction in leukocyte deformability could be measured in whole blood exposed to smoke in vitro or in vivo. Whole blood filterability, which largely reflects leukocyte deformability, was measured as the pressure developed by filtration of diluted whole blood through a micropore membrane. Whole blood filtration pressures did not change when blood was exposed to smoke in vitro or in venous blood after acute smoking in vivo. However, arterial blood sampled from chronic smokers during acute smoking showed a consistent reduction in leukocyte deformability associated with a small increase in plasma elastase. To assess whether these changes were induced by oxidants in cigarette smoke, we measured the levels of the antioxidant glutathione (GSH), erythrocyte (RBC) membrane fragility, and products of lipid peroxidation in plasma and RBC in blood exposed to smoke in vivo and in vitro. No change in RBC lipid peroxidation or membrane fragility could be detected after in vitro smoke exposure, possibly because of the high antioxidant capacity of the RBC. However, reduced blood GSH levels and increased levels of lipid peroxidation products were detected in plasma, reflecting oxidant stress. In contrast, we were unable to detect evidence of an increased oxidant burden in blood after acute smoking in vivo, in either arterial or venous blood samples. These data support the hypothesis that enhanced neutrophil sequestration in the lungs during smoking is due to a decrease in leukocyte deformability, which has the potential to increase the elastase burden in the lungs.