CF-1 female adult mice were given weekly sc injections of 20 mg symmetrical 1,2-dimethylhydrazine (DMH)-2HCl/kg body weight and killed at various intervals after commencement of the injection. [3H]thymidine (TdR) was given before the animals were killed. The histogenesis of colon neoplasms was investigated by means of autoradiographs prepared from sections of Epon-embedded descending colon, which were stained with periodic acid-Schiff reaction and iron hematoxylin. By 9 weeks after initiation of DMH treatment, the distal 5 cm of the colon became enlarged, the mucosa thickened, and the crypts were elongated and hyperplastic. In the hyperplastic crypts, the number of proliferating cells increased, but the distribution of these cells followed a previously discussed slow cut-off model of Cairnie et al. as for the normal crypts. Differentiation and transformation of epithelial cells occurred, but somewhat aberrantly. Hyperplasia of the crypts occurred diffusely, but neoplastic lesions that began to appear by 9 weeks after the intiial treatment were isolated. An isolated crypt from which a neoplasm developed was first repopulated by what appeared to be altered, undifferentiated "stem" cells. These cells did not differentiate, continued to divide, and eventually upon migration accumulated in the upper part of the crypts, where an earliest identifiable neoplastic lesion was observed. Once such a lesion was formed, it expanded in various directions, depending on the local environments, and formed a polypoid or discoid lesion. The biologic behavior of the neoplasm seemed to be determined by the downward progression of its leading edge. When it penetrated the muscularis mucosae, the neoplasm became highly invasive. In the murine model, the invasive adenocarcinomas were observed by 26 weeks after commencement of DMH treatment.