A patient whose neutrophils lack the glycoprotein gp-180 shows an increased susceptibility to bacterial infections. Neutrophils from this patient migrate abnormally both in vivo and in vitro. To examine the basis for this abnormality in migration, a study was carried out on the interaction of gp-180-deficient neutrophils with artificial surfaces and with human endothelial cell cultures. Compared with normal neutrophils. gp-180-deficient neutrophils showed decreased adhesion to cold-insoluble globulin-coated plastic surfaces, and their ability to spread on this substratum was greatly impaired. In contrast, gp-180- deficient neutrophils interacted in a normal fashion with endothelial monolayers, attaching to their surfaces and migrating between cell junctions to spread between the monolayers and the subjacent plastic. A normal interaction with endothelium in vivo was implied by the finding that the rise in the neutrophil count in response to epinephrine, an index of the marginated pool, was normal in the gp-180-deficient patient. We conclude that the abnormal function of gp-180-deficient cells is unlikely to be caused by a faulty interaction with the vascular endothelium. We postulate instead that these cells migrate poorly in vivo because of an abnormal interaction with extravascular connective tissue matrix constituents.