The cellular morphology, chromosomal structure, and tumorigenicity of two lines (B and J-13) of rat epithelial cells were examined serially during in vitro cultivation. The cells for such cultures were derived from the hepatic tissues of two 7-day-old male rats of the Donryu strain. The cultured cells were first inoculated into newborn syngenetic rats on the 641st day in vitro (80th subcultures) for line B, and on the 446th day (58 subcultures) for line J-13. The inoculated cells produced tumors with hemorrhagic ascites in rats after long latent periods, viz, 215-599 days in line B and 170-369 days in line J-13. All the tumors were undifferentiated hepatocarcinomas. The pleomorphism in shape and size of the cultured cells gradually became obvious with time of cultivation and was more pronounced in recultured tumor cells. Chromosomes of the culured cells were a normal diploid pattern until about the 200th day in vitro, but thereafter the modal chromsome number shifted to hypodiploid or hypotriploid via hypodiploid stages. The chromosome constitution of recultured tumor cells resembled that of inoculated cells in number distribution, but had changed to a more complicated karyotype. In experiments with line B, the same marker chromosome was detected in all tumor cells analyzed as had been present in inoculated cells.