A secretory proteinase inhibitor was isolated from the latex of green fruits of papaya (Carica papaya). The protein exhibited stoichiometric inhibition of bovine trypsin and alpha-chymotrypsin by the same site or overlapping binding sites. The complete covalent structure consisting of 184 amino acids and two disulfide bonds was determined by protein analysis. During the structural analysis, a procedure was established to separate very hydrophilic peptides by reverse-phase HPLC. The result revealed that the latex protein belongs to an extensively diverse plant protein family that includes inhibitors of serine, cysteine and aspartic proteases, a taste-modifying protein, wound responsive proteins, storage proteins, amylase inhibitors and even an oxidoreductase. In this superfamily, the latex proteinase inhibitor is most similar to the curious protein, miraculin, which makes sour food taste sweet. Two carbohydrate chains, each probably composed of (mannose)5, (xylose)1, (fucose)0-2, and (N-acetylglucosamine)2 residues, were attached to asparagine 84 and 90. Mass-spectrometric and compositional analysis suggested that they may represent a new class of plant xylose-containing carbohydrate chains with five mannose residues.