Adhesion of polymorphonuclear granulocytes (PMNs) in microvessels occurs in the presence of shear forces exerted by the blood flow. To model this in vitro, phorbol myristate acetate (PMA)-activated PMN were exposed to shear stress on cultured human umbilical vein endothelial cells (HUVECs) and on plastic dishes coated with bovine serum albumin (BSA). PMN adhesion to HUVECs averaged 36% of the total PMNs added and was reduced to 21% by shear stress of approximately 1.5 dynes.cm-2. On BSA, adhesion was reduced from 59% to 35%. Dextran sulfate (molecular weight 500,000) inhibited PMN adhesion in a dose-dependent manner when shear stress was applied. At a concentration of 1 mg.ml-1, inhibition was 72% on HUVECs and 76% on BSA. Half-maximal inhibition was reached at approximately 1 microgram.mL-1 dextran sulfate, corresponding to 2 nmol/L. Without shear stress, dextran sulfate had no effect on HUVECs and only a moderate effect on BSA. The murine monoclonal antibody (MoAb) 60.3, recognizing an epitope on the leukocyte adhesion glycoprotein CD18, inhibited PMN adhesion equally well with and without shear. A low dose of MoAb 60.3 enhanced the effect of dextran sulfate without shear stress. Flow cytometry (FACS) did not show inhibition of MoAb 60.3 binding to PMNs by dextran sulfate. These results indicate that a dextran sulfate-inhibitable adhesion process is important for PMN adhesion in the presence of shear stress.