Synthesis of [ 3H] methyl-di-(2-chloroethyl) amine (HN2) is described. The protection afforded by cysteine and related compounds against the lethal action of HN2 was studied in mice. It was found, that in those mice protected by cysteamine and cystamine before the injection of the tritiated nitrogen mustard, the excretion of the radioactivity was greater than in those that were not thus protected. The possible mechanisms of this protective action are discussed in relation to the theories of chemical inactivation and formation of mixed disulphides with the SH group of the tissue proteins.