This research was performed to establish a cell line from experimental bladder tumor and to discuss the biological characteristics of the cell line so established. Tissue cultures of epithelial cells were derived from a rat bladder cancer induced by BBN. The cells showed loss of contact inhibition and the phenomenon of piling up after several subcultures. Colonial cloning was used. The population doubling time of the wild strain and the colonial clones was about 30 h. The chromosomal mode ranged from triploid to tetraploid to tetraploid. Plating efficiency was well below 20%. Intraperitoneal backtransplantation into newborn Wister rats resulted in tumors in all cases. These tumors, in some parts, resembled primary transitional cell carcinoma. The major tumor cell groups, however, showed marked keratinization and the picture of squamous cell carcinoma. The nucleus/cytoplasm ratio and the numbers of nuclei, free ribosomes and intracytoplasmic microfibrils were increased. Dense microvillus arrangements characterized the electron microscopic picture. During the mitotic phase, the cells became large and globular whereas the microvilli were relatively short and were gathered profusely over the whole surface. Cells in the gap 1-synthetic phase developed lamellipodia and pseudpodia-like cytoplasmic processes and were polygonal in shape. Microvilli were present in the central part containing the nucleus, but their numbers were somewhat decreased and their height increased (scanning electron microscopy).