The study of the gamma-glutamyl transpeptidase (GTase) in colon of adult rats showed that in the sequence: duodenum, jejunum, ileum, cecum and colon, the colon has the lowest activity. There are, on the other hand, relatively small differences between GTase activities in the ascending, transverse and descending portions of the large intestine. GTase activity in the colon of neonatal rat is several times higher than in the colon of adult rats. Colon adenocarcinoma induced by 1,2-dimethylhydrazine were found to have a much higher GTase activity than the homologous normal tissue. Because these tumors resemble human colonic adenocarcinomas, it is suggested that the assay of GTase levels of human colon mucosa might be of potential value in the diagnosis of neoplastic changes.