Interleukin-8 (IL-8) induces diverse biological responses in neutrophils, including inhibition of adhesion to cytokine-activated endothelium, which we have termed the leukocyte adhesion inhibition (LAI) effect. Pretreatment of neutrophils with cytochalasin B abolished the LAI effect of IL-8, suggesting a microfilament-dependent mechanism. Interleukin-8 induced a rapid increase (≤ 15 s) in the polymerization of actin filaments in human neutrophils that was blocked by pretreatment with cytochalasin B. F-actin depolymerization occurred gradually at a rate inversely proportional to IL-8 concentration. This temporal pattern of actin polymerization-depolymerization was similar to that induced by other chemotactic factors such as C5a and N-formylmethionyl-leucyl-phenylalanine, which also exhibit a marked LAI effect, but the lipid mediators, leu-kotriene B₄ and platelet-activating factor, lack any significant LAI effect. Scanning confocal microscopy demonstrated that neutrophil actin microfilaments undergo a dramatic rearrangement prior to detachment of the neutrophil from a surface. We suggest that the ability of IL-8 and certain other leukocyte agonists to regulate the actin polymer network of neutrophils may play an important role in adhesive interactions with the vascular endothelium.