Résumé : A careful enzyme specificity analysis has revealed the presence of a typical gamma-glutamyltranspeptidase in the yeast Saccharomyces cerevisiae. The enzyme cellular level is low in the presence of NH4+ as a sole nitrogen source and rises when individual amino acids are used as nitrogen sources. The gamma-glutamyltranspeptidase appears to be repressed by NH+4 and escapes to the regulatory circuits under the control of glutamine and the glutamate-dehydrogenase . NH+4 complex. The transpeptidase cellular level is unaffected in mutants which have lost the general amino acid permease and specific systems for L-arginine and L-lysine. In contrast, a low enzyme level is observed when growing an apf mutant on urea; this mutant is most probably affected in a common element shared by all the amino acid permeation systems. Urea appears to be a nitrogen source which promotes a high transpeptidase level in the wild-type strain. The reported data are discussed in the light of the current theories about the intervention of glutathione metabolism in the translocation of amino acids.