A purified, GPI-linked receptor complex isolated from Manduca sexta midgut epithelial cells was reconstituted in planar lipid bilayers. CryIAa, CryIAc and CryIC, three Bacillus thuringiensis insecticidal proteins, formed channels at much lower doses (0.33-1.7 nM) than in receptor-free membranes. The non-toxic protein CryIB also formed channels, but at doses exceeding 80 nM. The channels of CrylAc, the most potent toxin against M. sexta, rectified the passage of cations. All other toxin channels displayed linear current-voltage relationships. Therefore, reconstituted Cry receptors catalyzed channel formation in phospholipid membranes and, in two cases, were involved in altering their biophysical properties.