In vivo, platelets associate with neutrophils at sites of hemorrhage or inflammation. In vitro, stimulated platelets bind to neutrophils in a Ca2(+)-dependent manner. GMP-140, an integral membrane glycoprotein found in secretory granules of platelets and endothelium, is rapidly translocated to the cell surface after cellular activation. It shares sequence similarity with two leukocyte adhesion molecules, ELAM-1 and a lymphocyte homing receptor. We have recently shown that neutrophils bind to purified GMP-140 in a Ca2(+)-dependent fashion, and that GMP- 140 participates in adhesion of neutrophils to activated endothelium. In this study we demonstrate that GMP-140 also mediates adhesion of neutrophils to stimulated platelets. Fixed thrombin-activated human platelets, but not unstimulated platelets, formed rosettes around neutrophils in the presence of Ca2+. The binding of platelets to neutrophils was inhibited by a monoclonal antibody to GMP-140 and by purified GMP-140. By promoting close cell-cell contact, GMP-140 may recruit both platelets and neutrophils to sites of tissue injury as well as modulate the function of each cell type by the other.