The ability of glycoprotein IIb-IIIa (GPIIb-IIIa) receptors on fully spread, surface-activated platelets to bind specific antibodies or ligands, singly or sequentially, and clear them toward cell centres has been established in several investigations. However, the basic mechanism involved in receptor-ligand translocation remains unclear. The present study has attempted to provide additional information by adding two different electron-dense probes simultaneously to surface-activated cells. Over a 5 min period of incubation small latex particles were cleared more rapidly from platelet margins to cell centres than simultaneously added particles of colloidal gold coupled to fibrinogen (Fgn/Au). Large and small latex spherules mixed together and added to spread platelets under the same conditions were moved at the same rate and concentrated together in the cell centres. Results of this investigation indicate that simple diffusion is unlikely to be the generating force for movement of receptor complexes on platelet plasma membranes.