a broad new class of reagents permits extremely sensitive and specific assay for , or chemical separation of , a broad range of biological and nonbiological substances . each reagent consists of a suspension of microscopic carrier material particles , each particle bearing tracer material - fluorescent , radioactive or otherwise - and a coating of biological homologue , that is , antibody , antigen , or portions or equivalents thereof , for the substance whose assay is desired . the latter substance if introduced into the suspension links the particles together in pairs or clumps , which may be sensitively and accurately detected by monitoring the tracer . the carrier is preferably partially hydrolyzed polyacrylamide resin , or in appropriate applications acrylic acid and other derivatives thereof , and other polymers including agar , and the coupling effected by covalent bonding . other embodiments , including various mechanical forms of carrier , for greater ease of handling and separation , are also described .