We report the development of a clonogenic assay for progenitor cells in transitional cell carcinoma of the bladder. Colony growth has been demonstrated from cells obtained both from surgical biopsies and from bladder barbotages. Electron microscopic and karyotypic evidence supports the contention that these progenitors represent a part of the population maintaining the tumor in vivo. Colony growth occurred in 9 of 11 surgical biopsy samples and in 6 of 6 bladder barbotage samples. Plating efficiency ranged up to 0.7%, and colony size was in some instances greater than 1000 cells. The assay appears potentially useful for analysis of the biology of human transitional cell carcinoma.