The mechanism of inactivation of a single-stranded RNA phage MS2 by AsA [L-ascorbate] was investigated as a part of the studies on the mechanism of inactivation of viruses by AsA. Investigations on the effects of O2, oxidizing or reducing agents, metals or chelating agents, and free radical scavengers on the inactivation of the RNA phage by AsA indicated that the free radical intermediates formed during the course of oxidation of AsA are responsible for the inactivation of the phage. H2O2 and D[dehydro]AsA themselves had no effect on the infectivity of the phage. Sucrose density gradient centrifugation analyses indicated that the reactive radicals react with the phage RNA to cause strand scissions in the RNA.