The ability of human epithelial cells derived from adult prostatic tissues to undergo clonal growth in culture was examined. In a previously described serum-free culture system, such cells exhibited a density-dependent growth requirement. It was found that raising the level of one of the constituents of the culture medium, bovine pituitary extract, to 100 micrograms/ml permitted excellent clonal growth when as few as 100 cells were inoculated/60-mm2 dish. Raising the levels of supplements other than pituitary extract (cholera toxin, epidermal growth factor, hydrocortisone, or insulin) did not produce this result. The average colony-forming efficiency of cells derived from primary or early passage cultures was approximately 25%. When single cell suspensions were prepared from tissue isolates and directly analyzed for clonal growth, colony-forming efficiencies were approximately 5%, perhaps indicating the proportion of stem cells with proliferative potential in the original isolates. The colony-forming efficiency of a cell population derived from cancer tissue was not significantly different from those of populations derived from normal tissues.