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Protection of bacteriophage T1 against x-rays was tested in the presence of concentrations of (NH(4))(2)SO(4) ranging from 10(-6)M to saturation (4.26 M). Survival of T1 in concentrations of 10(-6) to 10(-3)M after irradiation did not differ significantly from survival in distilled water after irradiation. From 10(-3)M to 10(-1)M there was a steep rise in survival, with a leveling off as the concentration approached saturation, giving over-all a 2,000-fold increase in survival. The mechanism of salting out protection in these experiments is apparently due chiefly to dehydration, which protects the virus particles against the indirect effects of x-irradiation. Postirradiation effects, tested by the inactivation of phage added to irradiated media, approach in magnitude the effects obtained by irradiation of the phage particles themselves in the various solutions. Filter paper adsorption analyses indicate a close correlation between concentrations of (NH(4))(2)SO(4), ability of the filter paper to adsorb phage, and protection against x-rays, both during and after irradiation.