Seeds of the common bean (Phaseolus vuggaris L.) contain proteinous α-amylase inhibitor (αAI) that inhibits insect and mamnalian α-amylase enzymes. Based on the specific inhibition of α-amylase enzylnes, common beans were classified into four different αAI types, αAI-1, 2, 3, and null type. This trait is controlled by a single locus, and alleles encoding the three αAI variants, αAI-1, 2, and -3, are codominant. Two different inhibitor proteins were purified from a common bean cultivar containing αAI-3. The two proteins are encoded at the same locus and designated αAI-3a and αAI-3b. αAI-3a inhibited the activity of porcine pancreatic α-amylase, while αAI-3b inhibited the activity of Mexican bean weevil larvae. N-terminal sequences and the specificity of αAI-3a and αAI-3b were very sinrilar to αAI-1 and αAI-2, respectively, indicating that the common bean classified als αAI-3 type contains both analogs of αAI-1 and a αAI-2.