Bovine serum albumin has been conjugated with kainylaminooxyacetylglycine to afford a multivalent kainylated protein called kainyl-bovine serum albumin (KA-BSA). This derivative, radiolabelled with 125I to more than 5000 Ci/mmol, was found to interact in the chick, goldfish and rat brain to specific membranous sites displaying the pharmacological properties attributed to the kainate sub-type of glutamate receptor. Measurements of the kinetics of association and dissociation of KA-BSA showed a quasi-irreversible binding with dissociation constants in the subpicomolar and nanomolar range. The chemical properties and the binding characteristics of KA-BSA suggest that it interacts mainly with kainate binding sites present in clusters in the membrane. Localization of the KA-BSA binding sites, by autoradiography in the chick cerebellum and by immunoperoxidase staining in the goldfish cerebellum, revealed an exclusive association with the molecular layer.