Five murine lymphoma cell lines were assayed for the content and activity of gamma-glutamyltranspeptidase (GTT). All lines [S49; L-12; 230-23-8 (C57 Black); 2M3 and RA3-2C2] contained detectable amounts of GGT. The specific activities of GGT were low and ranged between 1.2 and 2.3 mU/mg protein in cells growth in vitro. A highly malignant variant of the S49 line was also grown in vivo in BALB/c mice. This subline invariably produces both solid and ascitic tumors with infiltrations into the pancreas, liver and spleen. GGT levels in the tumor cells were low and independent of tumor type (solid, ascitic), location, passage number or inoculum size. Infiltrations of S49 tumor cells in liver and spleen were invariably GGT--negative as judged by histochemical examination. GGT activities in suspension cultures prepared from solid, as well ascitic tumors were low. Occasional high GGT activity of solid tumors was due to the presence of pancreas cells in them. The only host tissue significantly responding to the presence of tumors by elevated GGT levels was the liver. Compilation of data from this study and those of others clearly indicates that low GGT level is a typical property of tumors originating in the immune system.