Neutrophil-activating protein 1/interleukin 8 (NAP-1/IL-8)' was first identified as a 72 amino acid peptide secreted by monocytes in response to bacterial LPS, with the properties of activating and attracting polymorphonuclear leukocytes (PMN) in vitro (1). Subsequently several different types ofcells, including macrophages and endothelial cells, were found to synthesize NAP-1/IL-8 upon stimulation with TNF or IL-1. Sequence data indicate that NAP-1/IL-8 is homologous to a group of peptides including platelet basic protein, platelet factor 4, and macrophage inflammatory protein 2 . In vivo accumulation of PMN at sites of injection of NAP-1/IL-8 suggests that it may participate in the recruitment of PMN into inflamed tissue, and NAP-1/IL-8 has been identified in skin lesions of patients with psoriasis (1).Several lines of evidence indicate that the CD11/CD18 complex ofleukocyte adhesion receptors may participate in the adhesion of PMN to the vascular endothelium which is necessary for extravasation . mAbs specific for CD18 are effective in blocking the adherence of stimulated PMN to endothelium in vivo (2) and in vitro (3, 4), and patients genetically deficient in the CDII/CD18 complex exhibit poor recruitment of PMN to Rebuck skin windows (5). Here we describe the response of CD11/CD18 on PMN to NAP-1/IL-8, with particular emphasis on the numerically dominant member, CD11b/CD18 . This receptor not only recognizes ligands on endothelium but also binds to complement protein C3bi (6), fibrinogen (7), and LPS (8).