An epithelioid cell line (PS-1) has been established from a transitional cell cancer derived from human urinary bladder. Subcutaneous injection of the epithelioid cells into weanling athymic nude mice induced solid tumors histologically similar to the original tumor. A cell line was also established from a tumor induced in the athymic nude mouse (PS-1, T-1). Both cell lines exhibited essentially identical growth characteristics and formed a monolayer growth of epithelioid cells in culture. Electron microscopic studies confirmed epithelioid morphology. No fibroblastoid elements were observed. Chromosomal analysis revealed heteroploidy with persistent marker chromosomes; all cells contained a Y chromosome. The presence of tumor-specific antigen(s) in PS-1 cells was suggested by microcytotoxicity assays with peripheral allogeneic lymphocytes from other transitional cancer cell patients. Sera of urinary bladder cancer patients reacted with nuclear antigens of the established cells.