The binding properties of Bacillus thuringiensis toxins to brush border membrane vesicles of Dipel-resistant and -susceptible Ostrinia nubilalis larvae were compared using ligand-toxin immunoblot analysis, surface plasmon resonance (SPR), and radiolabeled toxin binding assays. In ligand-toxin immunoblot analysis, the number of CrylAb or CrylAc toxin binding proteins and the relative toxin binding intensity were similar in vesicles from resistant and susceptible larvae. Surface plasmon resonance with immobilized activated CrylAb toxin indicated that there were no significant differences in binding with fluid-phase vesicles from resistant and susceptible larvae. Homologous competition assays with radiolabeled CrylAb and CrylAc toxin and vesicles from resistant and susceptible larvae resulted in similar toxin dissociation constants and binding site concentrations. Heterologous competition binding assays indicated that CrylAb and CrylAc completely competed for binding, thus they share binding sites in the epithelium of the larval midguts of O. nubilalis. Overall, the binding analyses indicate that resistance to CrylAb and CrylAc in this Bt-resistant strain of O. nubilalis is not associated with a loss of toxin binding. (C) 2004 Elsevier Inc. All rights reserved.