If the content of serum in the culture medium of exponentially growing Chinese hamster cells is below optimum (15 percent), the doubling time and the resistance to x-irradiation of the cells are increased. In synchronously dividing populations the increase in doubling time is primarily caused by increase in duration of the postmitotic (G(1)) phase of the cells; this phase is relatively radiation resistant. The response of the cells growing synchronously is related quantitatively to the response of the cells dividing randomly.