This study was undertaken to systematically investigate the binding kinetics of platelet recruitment by monocytes relative to neutrophils in bulk suspensions subjected to shear as well as the molecular requirements of leukocyte-platelet binding. Hydrodynamic shear-induced collisions augment the proportion of monocytes with adherent platelets more drastically than that of neutrophils with bound platelets. These heterotypic interactions are further potentiated by platelet activation with thrombin or to a lesser extent by rnonocyte stimulation with N-formyl-methionyl-leucyl-pheiiylalanine (fMLP). Monocyte-platelet heterciaggregation increases with increasing shear rate and shear exposure time. Platelet P-selectin binding to monocyte P-selectin-glycoproteiii-ligand-I is solely responsible for maximal platelet adhesion to unstimulated monocytes in shear flow. However, the enhanced platelet binding to fMLP-treated nionocytes involves a sequential two-step process, wherein P-selectin-PSGL-1 interactions are stabilized by CD18-integrin involvement. Blocking platelet alpha(11b)beta(3) or monocyte 1-integrin function had no effect. This study underscores the preferential recruitment of platelets by monocytes relative to neutrophils in shear flow, and demonstrates that the shear environment of the vasculature coupled to the state of cell activation modulates the dynamics and molecular constituents mediating monocyte-platelet adhesion. (C) 2005 Elsevier Inc. All rights reserved.