Cysteinamine is about 5 times as effective as cysteine when given to mice intraven. in equimolar amts. prior to X-irradiation. However, max. tolerated doses of each give equivalent protection. Cysteinamine and cysteine appear to be additive in their protective effect against acute radiation lethality. This is suggestive of a similar mechanism of action. The observation of a postirradiation effect of cysteinamine in rats with the liver area shielded has not been verified.