Brush border membrane vesicles (BBMVs) were prepared from the 2(nd) instar larvae of Helicoverpa armigera. Binding of the activated Cry1Ac of Bacillus thuringiensis (Bt) toxin was shown by immunoblot. A 120-kDa protein was identified as a receptor for the Cry1Ac type delta -endotoxin. The aminopeptidase-N activity of BBMVs was measured as the hydrolysis Of L-leucine p-nitroanilide. The specific activity was 35 units/mg protein. The BBMV preparation also showed low level of alkaline phosphatase activity. Zn++ chelating agents 2,2 ' -dipyridyl and 1,10-phenanthroline inhibited aminopeptidase activity at 10 mm concentration, indicating the presence of zinc-dependent aminopeptidase in the brush border of H. armigera. The aminopeptidase activity was increased with increasing concentration of delta -endotoxin. The purified 120-kDa binding protein was N-terminally sequenced. The first 10-aminoacid sequence showed 60-77% similarity with human cysteine-rich secretory protein-1 precursor, inhibin alpha chain precursor. Salmonella flagellar hook protein and yeast carboxypeptidase S.