This paper extends previous observations on the interaction of 3-amino-1,2,4-triazole (amitrole) and riboflavin leading to the decomposition of amitrole to include the protective effect of amitrole on the photodestruction of flavins and flavoproteins. Photoactivation of radioactive amitrole in the presence of riboflavin causes the labeling of inert protein. Cupric ion, ascorbic acid, and oxygen can replace riboflavin and light in the destruction of amitrole and in the labeling of inert protein. The inhibition of fatty acid peroxidase previously reported by Castelfranco has been shown to be irreversible and to require the presence of a hydrogen peroxide generating system. The authors suggest a hypothesis for the mode of action of amitrole which depends upon the ability of this compound to interact with free radical generating systems, presumably by undergoing a one-electron oxidation.