Colon tumors of mice were induced with 1, 2‐dimethylhydrazine, N‐nitroso‐N‐methylurethane, methylnitrosourea, and 4‐methyl‐N′‐nitro‐N‐nitrosoguanidine. Of 82 transplantation attempts, four were successful, and the four tumors have been maintained as distinct tumor lines by subcutaneous transplantation. The tumors graded from II, adenocarcinoma, to IV, undifferentiated carcinoma. Volume‐doubling times varied over a three‐fold range, and metastatic potential ranged from 5 to about 100%. The tumors responded to treatment with some of the same agents found to be active in man. These included cyclophosphamide, 5‐fluorouracil, and certain nitrosoureas. Preliminary evidence suggests that these tumor lines may be useful as experimental models.