Gamma-glutamyltranspeptidase (GGT) was purified to electrophoretic homogeneity from the cell extract of H. pylori. The purified enzyme consisted of heavy and light subunits with molecular weights of 38 kDa and 21 kDa, respectively. N-terminal amino acid sequence of heavy and light subunits revealed that H. pylori GGT was processed into 3 parts for a signal peptide of 27 amino acid residues, a heavy subunit of 352 residues, and a light subunit of 188 residues during translation. The reaction rate for hydrolysis of ${gamma}$-GpNA was 84.4 ${mu}mol/min$ per milligram of protein, and that for the ${gamma}$-glutamyl transfer from ${gamma}$-GpNA to gly-gly was 23.8 ${mu}mol/min$ per milligram of protein. The apparent Km values of H. pylori GGT for ${gamma}$-glutamyl compounds were on the order of $10^{-3}$ to $10^{-4}$ M and those for acceptor peptides and amino acids were on the order of $10^{-1}$ to $10^{-2}$ M. The GGT protein kept approximately 80% of the initial enzymatic activity on incubation at $60^{circ}C$ for 15 min. The optimum temperature and pH for reactions of both hydrolysis and transpeptidation were $40^{circ}C$ and 9.0, respectively. The transpeptidation and hydrolysis reactions catalyzed by H. pylori GGT were strongly inhibited by L-Gln and moderately inhibited by L-Ala, L-Ser, ${eta}$-chloro-L-Ala, and L-Glu. These results demonstrated that the biochemical properties of H. pylori GGT are different from those of other bacterial GGTs. Further, H. pylori GGT might degrade glutathione in the gastric mucous layer of humans if the enzyme could be secreted in the bacterial niches.