A member of the Ascaris inhibitor family exhibiting anti-cathepsin G and anti-chymotrypsin activity was purified from the larval hemolymph of the honey bee (Apis mellifera). Three forms of the inhibitor, designated AMCI 1-3, were isolated using gel filtration and anion-exchange chromatographies followed by reverse-phase HPLC. The amino-acid analyses indicated that AMCI-1 and AMCI-2 have an identical composition whereas AMCI-3 is shorter by two residues (Thr, Arg). All three forms contain as many as 10 cysteine residues and lack tryptophan, tyrosine, and histidine. The sequence of the isoinhibitors showed that the major form (AMCI-1) consisting of 56 amino-acid residues was a single-chain protein of molecular mass 5972 Da, whereas the other two forms were two-chain proteins with a very high residue identity. The AMCI-2 appeared to be derived from AMCI-1, as a result of the Lys24-Thr25 peptide bond splitting, while AMCI-3 was truncated at its N-terminus by the dipeptide Thr25-Arg26. The association constants for the binding of bovine alpha-chymotrypsin to all purified forms of the inhibitor were high and nearly identical, ranging from 4.8 x 10(10) M-1 for AMCI-1 to 2.7 x 10(9) M-1 for AMCI-3. The sensitivity of cathepsin G to inhibition by each inhibitor was different. Only the association constant for the interaction of this enzyme with AMCI-1 was high (2 x 10(8) M-1) whereas those for AMCI-2 and AMCI-3 were significantly lower, and appeared to be 3.7 x 10(7) M-1 and 4.5 x 10(6) M-1, respectively. The reactive site of the inhibitor, as identified by cathepsin G degradation and chemical modification, was found to be at Met30-Gln31. A search in the Protein Sequence Swiss-Prot databank revealed a significant degree of identity (44%) between the primary structure of AMCI and the trypsin isoinhibitor from Ascaris sp (ATI). On the basis of the cysteine residues alignment, the position of the reactive site as well as some sequence homology, the cathepsin G/chymotrypsin inhibitor from larval hemolymph of the honey bee may be considered to be a member of the Ascaris inhibitor family.