The effect of flow on the adhesion of neutrophils (PMNL) to vascular endothelium was investigated using a parallel plate flow chamber. Human umbilical vein endothelial cells (HUVEC) were cultured on a glass slide which was fitted onto the flow cell. Formyl-methionyl-leucyl-phenyl-alanine (FMLP), a chemotactic tripeptide, interleukin-1 (IL1), lipopolysaccharide (LPS), and thrombin were used to stimulate the PMNL and HUVEC in order to model the effect of inflammatory mediators on leukocyte adhesion under controlled flow conditions. PMNL adhesion to HUVEC monolayers was measured over a range of wall shear stresses estimated to be representative of flow conditions in the microcirculation. HUVEC monolayers were treated with interleukin-1 (IL1, 2U/ml, 4 hours) preceding the experiment. At 2.0 dynes/cm\sp2 wall shear stress, 371 ± 25.8 PMNL/mm\sp2 (mean ± SEM) adhered to IL1-treated HUVEC and 28 ± 2.9 PMNL/mm\sp2 adhered to control HUVEC after ten minutes of flow (p < 0.01 on the adhesion ratio, n = 5). At 3.0 dynes/cm\sp2 wall shear stress, 10.2 ± 3.8 PMNL/mm\sp2 adhered to IL1-treated HUVEC and 6.8 ± 3.5 PMNL/mm\sp2 adhered to control HUVEC (n = 5). PMNL adherence to IL1-treated HUVEC decreased significantly at 3.0 dynes/cm\sp2 compared to adherence at 2.0 dynes/cm\sp2 (p < 0.005). The CD18 family of leukocyte glycoproteins has been identified as a mediator of a number of adhesive interactions crucial to the inflammatory response. Incubation of PMNL with TS1/18 (anti-CD18) did not inhibit PMNL adhesion to IL1-treated HUVEC at 2.0 dynes/cm\sp2. TS1/18 inhibited migration of PMNL beneath IL1-treated HUVEC monolayers by 82 ± 6.8%. In flow experiments with CD18-deficient PMNL, no transendothelial migration was observed. The effect of FMLP (10\sp−8 M) on PMNL adhesion to untreated HUVEC was investigated at wall shear stresses ranging from 0.25 to 2.0 dynes/cm\sp2. FMLP stimulation did not significantly increase PMNL adherence at shear stresses above 0.5 dynes/cm\sp2. It was possible using a flow system to demonstrate that the initial attachment (or margination) of PMNL does not involve the same membrane associated adhesive proteins as does subsequent migration beneath HUVEC monolayers. Additionally, by controlling the level of shear force, it was possible to distinguish CD18/ICAM-1 mediated attachments to HUVEC from CD18-independent ones. The CD18/ICAM-1 dependent component appears to contribute to attachment at wall shear stresses below 1.0 dyne/cm\sp2, while the CD18/ICAM-1 independent component appears to form stronger or more numerous bonds that mediate adhesion at higher wall shear stresses. These observations indicate that local blood flow rates in the vasculature can play an important role in regulating the margination and attachment of leukocytes to the blood vessel wall.