Eight-week-old mice of 3 sublines of strain C57BL/6 were given s.c. injections of 1,2-dimethylhydrazine (DMH), once weekly for 10 weeks. The highest incidence (85%) of colorectal tumors occurred in C57BL/6N mice. Colorectal tumors occurred in 43% of C57BL/6J mice, while only 3 (10%) C57BL/6Ha mice developed these tumors. Possible factors responsible for the differential susceptibility of 3 sublines of C57BL/6 mice to the induction of colorectal tumors by DMH are discussed.