One trypsin inhibitor (I) from zucchini seeds and two (I and III) from squash seeds were isolated by ammonium sulphate salting out and chromatography on SP-Sephadex C-25 and immobilized trypsin. The inhibitors have the same molecular mass, about 3300, contain 29 amino acids, have three disulphide bridges, and lack Thr, Phe and Trp. The isoelectric point of the zucchini inhibitor I and squash inhibitor I is of 5.6, whereas that of the squash inhibitor III--of 8.3. Arg is the N-terminal amino acid of all three inhibitors. On modification of the Arg residues with 1,2-cyclohexanedione both squash inhibitors become inactivated. The zucchini inhibitor I becomes inactivated on acetylation of free amino groups. In all three inhibitors, on enzymatic modification with trypsin at pH 3.2, Ile appears as a new N-terminal amino acid. On this basis it can be supposed that Arg-Ile form the reactive site of the trypsin inhibitors from squash seeds, and Lys-Ile of the zucchini seed inhibitor.