Cry1Ie is encoded by cry1Ie, a gene that is silent in Bacillus thuringiensis strains but can be over-expressed in Escherichia coli. A protease-resistant core form of Cry1Ie with a molecular weight of approx. 55 kDa was found among the products of trypsin digestion. The oligomer and monomer of the protease-resistant core form were purified separately. The oligomer fraction contained a small quantity of dimer and a large amount of aggregates larger than tetramers. The 50% lethal concentrations (LC50) of the full-length Cry1Ie and both the monomer and oligomer fractions of the protease-resistant core form against Plutella xylostella were 14, 21 and 1400 mu g/ml, respectively.