A new enzyme from Hirudo medicinalis capable of splitting γ-glutamyl-p-nitroanilide and Glu Lys-(N 6 -γ-glutamyllysine) (isopeptidic bond between the ε-amino group of lysine and the γ-carboxylic group of glutamic acid) isopeptide bonds was purified. The protein was partially sequenced at the amino acid level, and the complete nucleotide and amino acid sequences were determined after cDNA cloning. The new enzyme has more than 60% similarity at the amino acid level to vertebrate γ-glutamyl transpeptidase (γ-GT). According to the cDNA, the new protein has a molecular mass of 65521 Da and a length of 600 amino acids.