The ability of cell populations to survive in the aggregate form was compared to colony formation in soft agar and tumorigenicity in nude mice. Nontumorigenic human osteogenic sarcoma (HOS) cells, which formed colonies in soft agar, could not survive in the aggregate form. Tumorigenic HOS cell lines, which also formed colonies in soft agar, survived and proliferated in the aggregate form. Other cell types were tested with the same results. This approach, based on cell survival in the aggregate form, may provide an additional, reliable method for predicting the tumorigenic status of a cell population.