Human carcinoma tissues were grown in culture for 2-4 wk using the 2-layer soft agar technique. All cultures that showed growth of tumor cell colonies also showed well-preserved, apparently healthy tumor cells lying singly. These cells showed neither proliferative capacity nor necrosis or mophologic degeneration during the time in soft agar. Morphologic criteria seem to be poor indicators of tumor cell proliferative potential, at least in the short term. The method of soft agar tumor clonogenic cell culture itself provided a direct measure of tumor cell proliferative capacity, i.e., the formation of colonies from single tumor cells. This may be valuable in directly assessing the presence of viable tumor cells in biopsy specimens taken after therapy, thus guiding further patient therapy.