Continuous monolayers of activated platelets support selectin-mediated rolling and integrin-mediated immobilization of flowing neutrophils. Because platelets attached to the vessel wall in vivo may not be confluent, we investigated the ability of sparsely adherent platelets to capture flowing neutrophils that were unstimulated or activated with N-formyl-methionyl-leucyl-phenylalanine (fMLP) peptide (10(-7) mol/L). Different degrees of deposition of platelets were obtained by perfusing heparinized blood for varying periods through glass capillaries coated with collagen type III. Perfusion for 10 to 180 seconds gave approximately 1% to approximately 10% coverage of the surface with platelets. When purified neutrophils were subsequently flowed over the platelets at a wall shear stress of 0.1 Pa, they formed repeated short-lived attachments to individual or small groups of platelets. Increasing surface coverage with platelets was associated with increasing numbers of neutrophils adherent at any time, decreasing the overall rate of motion of adherent neutrophils and increasing the stability of adhesion (eg, proportion remaining with time). The net effect was to actually decrease the flux of adherent cells (number of adhesive interactions seen per unit area per unit time), indicating that parameters based on through-flow of adhesive cells must be interpreted with caution. When neutrophils were preactivated with fMLP they formed stable, stationary attachments. The flux of adherent activated cells was low, although the number of cells adherent remained high, reinforcing the conclusion that assays of the adhesion of flowing cells must carefully characterize the different types of adhesive behavior. Overall, small numbers of platelets can support short-lived attachments of flowing neutrophils and allow immobilization of activated cells and may amplify inflammatory processes thereby.