ICR mice were injected with 20 mg/kg of 1,2-dimethylhydralazine(DMH) subcutaneously per week for 20 weeks to induce colorectal carcinoma. Two different regimens were used for β-carotene(β-C) administration in this model.Mice in the first group was given β-C(15 mg/kg) twice per week from the beginning of the experiments,and the other group started to take β-C(15 mg/kg) twice per week at the 17th week of the test.This experiment lasted 26 weeks.The results showed a decrease in the incidence of colorectal carcinoma and the number of tumors as well as the diminish of the SIL-2 receptor in the first group. The latter group only showed the decreasing number of tumors. In another test,mice were given β-C 5 mg/kg/d by mouth for 7 days,followed by injction of DMH s. c. Twenty-four hours later,the frequency of micronuclei of the large bowel epithelium decreased significantly.