Based on the results of mapping of ggt, eight strains were selected from a gene library of E. coli. One of the strains harboring pLC9-12 was found to show 14 times higher gamma-glutamyltranspeptidase activity per cell than the wild type strain. The ggt was subcloned to the BamHI site of pUC18 and the recombinant plasmid pSH101 was obtained. Ggt- phenotype of gamma-glutamyltranspeptidase-deficient mutants was complemented by pSH101. The specific activity of the enzyme in cells harboring pSH101 was 37-fold higher than that in the wild type cells. gamma-Glutamyltranspeptidase was isolated from the periplasmic fraction of the cells by simple two steps and crystallized.