Human urinary bladder cancer cells (31) were transplanted s.c. into congenitally athymic nude mice. Subcutaneous growth was obtained in 8 at the initial transplantation. Only 1 of these 8 successful transplants [NKPB-8 cells], which was derived from squamous cell carcinoma, was serially transplanted successfully and was characterized by high thromboplastic and low fibrinolytic activities, which were virtually constant throughout the serial passages. This strain will provide a useful model system for studying anticancer therapies.