The F11 receptor (F11R) (a.k.a. Junctional Adhesion Molecule, JAM) was first identified in human platelets as a 32/35 kDa protein duplex that serves as receptor for a functional monoclonal antibody that activates platelets. We have sequenced and cloned the F11R and determined that it is a member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. The signaling pathways involved in F11R-induced platelet activation were examined in this investigation. The binding of M.Ab.F11 to the platelet F11R resulted in granule secretion and aggregation. These processes were found to be dependent on the crosslinking of F11R with the Fc gammaRII by M.Ab.F11. This crosslinking induced actin filament assembly with the conversion of discoidal platelets to activated shapes, leading to the formation of platelet aggregates. We demonstrate that platelet secretion and aggregation through the F11R involves actin filament assembly that is dependent on phosphoinositide-3 kinase activation, and inhibitable by wortmannin. Furthermore, such activation results in an increase in the level of free intracellular calcium, phosphorylation of the 32 and 35 kDa forms of the F11R, F11R dimerization coincident with a decrease in monomeric F11R, and association of the F11R with the integrin GPIIIa and with CD9. On the other hand, F11R-mediated events resulting from the binding of platelets to an immobilized surface of M.Ab.F11 lead to platelet adhesion and spreading through the development of filopodia and lammelipodia. These adhesive processes are induced directly by interaction of M.Ab.F11 with the platelet F11R and are not dependent on the Fc gammaRII. We also report here that the stimulation of the F11R in the presence of nonaggregating (subthreshold) concentrations of the physiological agonists thrombin and collagen, results in supersensitivity of platelets to natural agonists by a F11R-mediated process independent of the Fc gammaRII. The delineation of the two separate F11R-mediated pathways is anticipated to reveal significant information on the role of this cell adhesion molecule in platelet adhesion, aggregation and secretion, and F11R-dependent potentiation of agonist-induced platelet aggregation. The participation of F11R in the formation and growth of platelet aggregates and plaques in cardiovascular disorders, resulting in enhanced platelet adhesiveness and hyperaggregability, may serve in the generation of novel therapies in the treatment of inflammatory thrombosis, heart attack and stroke, and other cardiovascular disorders.