The protective action of a series of organic sulfur compounds and amino acids has been investigated by using glycine and glycylglycine as substrates. A linear relationship is shown to exist between the reciprocal of the G-m value for the substrate in the presence of the protecting agent, and the concentration of this protecting agent. The "protecting activity" of the agents, expressed as the ratio of the rate constants for the reaction of radicals with the protecting agent and the substrate, is determined.