The percent of adult (○, •) and neonatal (□, ▪) neutrophils initially attached to CHO cells expressing P-selectin in the absence of shear stress that remain interacting as shear stress is then applied and increased. Neutrophils are allowed to settle onto the monolayer for 2 minutes, at which point the flow is begun and increasing shear stress is applied every 20 seconds. The number of neutrophils that remain attached in the last 10 seconds before the next step up in shear stress is compared with the number of cells that had originally settled (percentage of interacting cells remaining). Neutrophils were left untreated (•, ▪) or were preincubated with anti–L-selectin MoAb, DREG 56 (○, □). Data are expressed as the mean ± SEM from 7 to 14 experiments.