The selectins are important receptors in cell-cell interaction involving circulating neutrophils and monocytes in the blood and their association with endothelium and activated platelets in the sites of tissue injury. P-selectin mediates the interaction of platelets and endothelial cells with leukocytes (1, 2). Following tissue injury, leukocytes are known to roll along the vessel wall. Lawrence & Springer (3), using purified P-selectin, demonstrated that P-selectin mediates this rolling phenomenon; work on the P-selectin null mouse (4) confirms that leukocytes in vivo become associated (“roll”) on the stimulated vasculature of the normal mouse but not the P-selectin null mouse. Once the leukocyte is associated with the vessel wall, other adhesion molecules such as integrins and members of the Ig superfamily play a role in immobilizing the leukocyte in the area of tissue injury. In summary, it currently appears that the selectins play a critical role in inflammation (5) and P-selectin in particular plays a role in thrombogenesis (6).