Several Cry1Ac binding proteins from midgut of Helicoverpa armigera were purified using toxin-affinity chromatography. Enzyme assays showed that the purified proteins had strong aminopeptidase activity. The N-terminal sequences confidently identified a 124-kDa binding protein as an aminopeptidase N (APN), and some similarity suggests that a 162-kDa binding protein may also be an APN. Two minor binding proteins were not characterized.