Five tumor-producing cell lines were established from explant cultures of adult C57BL mouse submandibular gland. Four lines were from cultures treated for 24 hr on Day 4 of culture with 7,12-dimethylbenz(a)anthracene. Three of these gave rise to adenocarcinomas after transplantation into syngeneic mice; the fourth produced tumors with carcinomatous and sarcomatous areas. The fifth cell line was derived from an untreated culture and gave rise to adenocarcinomas. A series of four well-defined morphological stages occurred in the cultures before tumor-producing cell lines were established. In Stage I (0 to 30 days) there was an outgrowth of epithelium; in Stage II (30 to 70 days) ductal differentiation occurred in some epithelium; in Stage III (70 to 100 days) small, slowly proliferating foci developed either from the ducts or from flat epithelial areas. In Stage IV (over 100 days) the proliferation rate in some of the foci increased, and the cells became more irregular. The cells could not be transferred easily until about 150 days, after which time they were tumor producing. Neoplastic transformation occurred between 158 and 240 days in the treated cultures and at 325 days in the untreated culture.