A cell line derived from human urinary bladder carcinoma, designated T24, was established in vitro. The growth of T24 cells in tissue culture was characterized by a disorderly pattern of growth in one or more layers and by mixed epithelioid-fibroblastoid morphology. The generation time of T24 cells was 19 h. The cells had a hypotetraploid stemline with marker chromosomes. The malignant character of the line was verified by inoculation of cell suspension into hamster cheek pouch. The presence of tumour-specific antigen (TSA) in T24 cells was demonstrated by a micromodification of the cytotoxicity test with autochthonous leukocytes and serum from the donor of the tumour. The TSA was also well detectable by the reaction with allogeneic leukocytes from tumour-bearing patients and persisted during a 30-month culture period. The permanent presence of TSA in this carcinoma cell line suggests that the TSA is a genetically determined characteristic of T24 cells.