A purse-string suture was put into the rat's cecum to form a "diverticulum." When the thread cut this stitch, the resultant extensive necrotic zone healed for a long time. The presence of a foreign body (ligature) provided a permanent source of injury to the cecal mucosa. The lesions caused an increase in [3H]thymidine-labeled epithelial cells in the adjacent tissue detected by means of microautoradiographs. A postinjury injection of 1,2-dimethylhydrazine resulted in a marked increase in the rate of cecal tumor incidence (from 23 +/- 2.8% under ordinary conditions to 87 +/- 6% and 96 +/- 4% in different experimental series). The rise in tumor incidence following injury may be due to the entry of a greater number of stem cells into the mitotic cycle at which stage they seem to be responsive to carcinogenic influences.