Fluoresbrite carboxylate microspheres are fluorescent monodispersed polystyrene microspheres that have carboxylate groups on their surfaces which can be activated for the covalent coupling of proteins. Polysciences Fluoresbrite particles are used worldwide in phagocytosis and neural retrograde transport studies, and as markers for cell bound antigens. These microspheres are packaged as 2.5% aqueous suspensions.