The role of leukocyte migration induced by the substrate-bound attractants is obscured by the inability of standard methods for the study of leukocyte migration to dissociate chemotaxis and haptotaxis, migration in response to soluble and surface-bound gradients of attractants. Here we show that the gradient of neutrophil attractant/activation protein-1 (interleukin-8, NAP-1/IL-8) induces directed in vitro neutrophil migration when bound to the polycarbonate filter. In addition, we suggest that haptotaxis is responsible for neutrophil migration induced by NAP-1/IL-8 in standard Boyden-type chemotaxis assays and, in light of the ability of NAP-1/IL-8 to bind to the endothelial cell surface and the extracellular matrix, could also be of great significance in vivo.