In previous work (Conforti et al, Blood 80:437, 1992), we have shown that integrins in endothelial cells (EC) are not polarized to the basal cell membrane, but are also exposed on the apical cell surface, in contact with blood. Therefore, endothelial integrins might be available for binding circulating plasma proteins. However soluble plasma vitronectin (vn) bound very poorly to EC apical surface and this interaction was unaffected by Arg-Gly-Asp (RGD) peptides or an anti- alpha v beta 3 serum. In contrast, beads (diameter, 4.5 microns) coupled with plasma vn associated to EC apical surface in a time- and concentration-dependent way. Addition of antibodies directed to vn, alpha v beta 3, and RGD-containing peptides blocked the interaction of vn beads with EC. In contrast, heparin and antibodies directed to alpha v beta 5 and beta 1 integrin chain had no effect. Beads coupled with Gly-Arg-Gly-Asp-Ser-Pro bound to the EC surface, but not those coupled with Gly-Arg-Gly-Glu-Ser-Pro. This interaction was blocked by alpha v beta 3 antibodies and RGD peptides, but not by alpha v beta 5 antibody. Overall, these results indicate that luminal alpha v beta 3 retains its binding capacity for surface-linked vn and RGD-containing ligands, but binding is observed only when the ligand is offered in a clustered, multivalent form. We propose that when vn or RGD-containing proteins are bound to circulating cells, they can act as bridging molecules by promoting adhesion of the cells to the endothelium via apical integrins.