[en] Viral DNA was irradiated at 254 nm in solvents of different ionic strengths and assayed for loss of ablility to transfer competent host cells. Two solvent systems were used: potassium phosphate buffer, pH 7.2, and standard saline citrate. DNA irradiated in the single-stranded form and then renatured was found to be more sensitive to UV inactivation than DNA irradiated in the native state. When native DNA was irradiated at room temperature, the sensitivity to biological inactivation increased as the ionic strength was decreased. For both buffer systems, a plot of logarithm of the survival curve slope versus logarithm of the cation concentration is linear for cation concentrations below 0.1 M. (Auth.)