Tragacanth gum was administered at dietary levels of 0 (control), 0.625, 1.25, 2.5, and 5.0% to groups of 10 male and 10 female B6C3F1 mice for 13 wk. There were no treatment-associated effects regarding clinical signs, body or organ weights, and urinalysis or hematology data. Significant dose-related, but slight, elevations of plasma gamma-glutamyl transpeptidase (GGT) level were observed in all treated animals except the 0.625% females. Single or small numbers of tiny nodules were observed on the luminal surface of the forestomach in 4 males of the 5.0% group, 2 males of the 2.5% group, and 1 male each from the 1.25 and 0.625% groups. Histopathologically, they were diagnosed as squamous-cell hyperplasia. To investigate the nature of these gross lesions, tragacanth gum was fed to groups of 30 male mice at the dietary level of 5.0% for periods of up to 48 wk; 20 males served as controls. There were no treatment-related increases of plasma GGT levels at wk 24 and 48. Although squamous-cell hyperplasias were seen in 2 out of 10 mice at wk 24, none of these proliferative lesions were apparent at wk 48, after either chronic exposure or 24 wk on basal diet. Furthermore, the levels of DNA synthesis in forestomach epithelium as measured by 5-bromo-2-deoxyuridine (BrdU) immunohistochemistry were comparable to control values at wk 24 and 48. Thus, the oral toxicity of tragacanth gum to B6C3F1 mice was concluded to be negligible.