Poly-trans-[(2-carboxyethyl) germasesquioxane] (Ge-132) was administered at dietary levels of 0 (control), 0.3, 0.8 and 2.5% to groups of 20 male and 20 female CB6F1-Tg rasH2 (rasH2) mice for 26 weeks. As a positive control, 10 rasH2 mice of each sex received a single intraperitoneal injection of 75 mg/kg N-methyl-N-nitrosourea (MNU). There were no differences in survival between Ge-132-treated groups and the control. Loose stool, increase water intake, and dilatation of the cecum were evident in both male and female 2.5% groups; however, there were no histopathological abnormality found in the cecums of these mice. There was no significant Ge-132 treatment-related increase in the incidence of any neoplastic lesions compared to negative control. In the positive control MNU groups, malignant lymphomas and squamous cell papillomas of the forestomach frequently occurred. Thus, the experimental system employed showed clear negative results for induction of tumors due to Ge-132 administration, indicating the absence of Ge-132 carcinogenicity in mice.