The initial inoculum level of Tetrahymena in a chemically defined medium determines whether the cells are capable of exponential growth. Below 750 cells ml−1, the cells fail to go into exponential growth and will die within about 20 hours. By adding certain growth stimulants, death can be postponed and the cells begin to grow after a delay which depends on the intensity of the signal. The implication is that autocrine growth factor expression might be required for cells to grow, and that these stimulants either assist its production or lower the cell threshold to its action. The findings in Tetrahymena are reviewed, and the advantages of having a cell system in which all the components of the medium can be carefully controlled is recognised.