MB49 cells are derived from C57BL/Icrf-a’ mouse bladder epithelial cells that were transformed by a single 24-hour treatment with the chemical carcinogen 7, 12-dimethylbenz[a]anthracene (DMBA) on the second day of a long term primary culture (1). Transformed cells transplanted into syngeneic mice were shown to generate carcinomas (1). While of male origin, karyotype analyses indicate the loss of the Y chromosome in 100% of the cells analyzed (2). This abnormality is a frequent early event in human bladder cancer.