When mammalian cells are treated with alkali of pH at about 12, the cells are lysed and the released DNA starts to uncoil. This process of DNA strand separation is accelerated if the cells have been exposed to ionizing radiation, and the effect is clearly detectable in the dose range 10–100 rads. The rate of strand separation is also influenced by temperature and ionic strength of the alkaline solution. The kinetics of DNA strand separation in alkali is studied for three conditions in terms of ionic strength and temperature, chosen in such a way that the effect of irradiation may conveniently be studied in the dose range 10 rads to 20 krads. The accelerating effect of ionizing radiation on DNA strand separation is probably due to DNA strand breakage and the technique described is thus a sensitive method of studying such damage to DNA. A model for the strand-separation process, based on the assumption that strand breakage causes the accelerating effect, is proposed and found to describe the experimental data adequately.