'y-Glutamyl transpeptidase, an enzyme that catalyzes y-glutamyl transfer from y-glutamyl compounds to amino acid and peptide acceptors, and which is known to be localized in the membranes of many epithelial cells, was found in a variety of lymphoid cells. The lymphoid cell enzyme is located on the cell surface and exhibits substantially the same substrate specificity as the enzyme found in epithelial cells. Lymphocyte Cultures. Human peripheral blood lymphocytes were obtained from normal subjects by the Ficoll-Hypaque method (7). They were further purified from the adhering cells by passing through a nylon-wool column, and the bone marrow-derived (B-) and thymus-derived (T-) cells were separated by the sheep erythrocyte resetting method (8). The lymphoc4ytes which did and did not form rosettes with sheep erythrocytes are referred to as T-lymphocytes and B-lymphocytes, respectively. Mouse (CBA strain; male, 6-12 weeks of age), rat (Sprague-Dawley; male, 250 g), and guinea-pig (strain 13; male, 250 g) lymphocytes were obtained from spleens and lymph nodes as described (9). The erythrocytes were selectively lysed by incubation in a buffer containing 0.15 M NH4Cl, 0.017 M Tris-HCl (pH 7.2) at 00 for 10 min. Mouse, rat, and guinea pig lymphocytes (2 X 106 cells per ml) and human peripheral blood lymphocytes (1 X 106 cells per ml) were cultured in RPMI 1640 medium containing 5% fetal calf serum (heat inactivated) and supplemented with penicillin (100 units/ml) and streptomycin (100 Mug/ml) at 370 in an atmosphere of 95% air-5% CO2.In the [3H]thymidine incorporation studies, aliquots (1 ml) of cell cultures were incubated for 2 hr with [3H]thymidine (2.5 1.Ci) and isotope incorporation into 5% trichloroacetic acidinsoluble material was measured (9).