The in vivo micronucleus test using mouse colonic epithelial cells was evaluated as the 11th collaborative study organized by the Collaborative Study Group on the micronucleus test (CSGMT) with three model chemicals that were known to induce chromosome damage in mouse colonic cells. Five laboratories participated in this validation study. All three model chemicals, i.e. 1,2-dimethylhydrazine dihydrochloride (1,2-DMH), N-methyl-N-nitrosourea (MNU), and mitomycin C (MMC), induced micronucleated colonic epithelial cells in a 4-day exposure protocol in all participating laboratories. We confirmed that the present single cell suspension method could be used to detect the model chemicals as micronucleus inducers in mouse colonic epithelial cells. Advantages of this method are that experiments are easy to perform and that intact cells can be analyzed. The present study suggested that the colon micronucleus assay proposed here is useful for mechanistic studies of colon carcinogenesis.