Infusion of complement fragments induces rapid sequestration of neutrophils within pulmonary capillaries. This study examined the mechanisms through which this sequestration occurs, as well as the effect of complement fragments on the expression of L-selectin and CD11/CD18 using ultrastructural immunohistochemistry. Studies using anti-P-selectin antibodies, fucoidin, L-selectin-depleted neutrophils, and anti-CD18 antibodies showed that selectins and CD18 were not required for neutrophil sequestration. However, maintaining the sequestered neutrophils within the pulmonary capillaries required both L-selectin and CD11/CD18. Neutrophils in the pulmonary capillaries of rabbits given complement fragments expressed 72% less L-selectin and 98% more CD11/CD18 than did those in rabbits given saline. Shedding of L-selectin occurred preferentially from the microvillar processes of the plasma membrane rather than from the flat intervening regions. About 28% of L-selectin still remained on intracapillary neutrophil membranes after 15 min and was likely available for binding. Shedding of L-selectin appeared slower in vivo than in vitro. These studies indicate that neutrophil sequestration induced by complement fragments requires at least two sequential steps, one that does not require recognized adhesion molecules followed by a second that requires L-selectin and CD11/ CD18.