Changes in the membrane permeability of Manduca sexta midgut brush-border-membrane vesicles (BBMV) after addition of Bacillus thuringiensisδ-endotoxins were studied using osmotic swelling experiments, volume changes being monitored as the change in 90° light scattering. Typically, control BBMV exhibited limited permeability for sucrose and salts (KCl), while being permeable for urea and glucose. The action of δ-endotoxin was examined using proteolytically activated Cry-IA(c) and CryIB toxins. CryIA(c) produced a marked change in the solute permeability of M. sexta BBMV, significant effects being observed at 3.75 pmol/mg BBMV. The permeability change was relatively non-selective with cations, anions and neutral solutes all traversing the membrane to an increased extent in the presence of CryIA(c). In contrast, the CryIB toxin had no effect on BBMV permeability.