During inflammation, neutrophils migrate out of blood vessels into the surrounding tissues. A number of studies in vivo have indicated that chemotactic factors, particularly fragments derived from the fifth component of complement, are able to mediate this emigration. The ability of C5 fragments to induce migration of neutrophils through the endothelium and internal elastic lamina of the rabbit carotid artery in vivo was investigated. Application of C5 fragments to the adventitial surface of the artery, in combination with either mechanical trauma or the simultaneous administration of PGE2, resulted in neutrophil adherence to the endothelium and migration into the vessel wall. The demonstrated ability of neutrophils to interact with the endothelial cells of large arteries validates to some extent the use of readily available large vessel endothelium for the in vitro examination of neutrophil-endothelial cell adherence, the initial step in the process of emigration. A sensitive, reproducible assay has been established which allows the assessment of neutrophil adherence to human umbilical vein endothelial cell monolayers. Adherence could be stimulated in vitro in a dose-response fashion by the addition of chemotactic factors, either fMetLeuPhe or C5 fragments. The synergistic effect of PGE2 observed in the in vivo transmigration system was not apparent in the in vitro adherence assay. It is possible that PGE2 does not participate at the level of the initial neutrophil-endothelial adhesive interaction. Pretreatment of the endothelium with the chemotactic factors failed to induce increased adherence and may suggest that the effect of chemotactic factors on adherence is primarily, if not entirely, limited to the neutrophil.