The authors examine a method for the synthesis of S-(3-aminobutyl)isothiourea dihydrobromide and its cyclization product, 2-amino-4-methyl-5,6-dihydro-4H-1,3-thiazine. The yield and properties of the synthesized compounds are presented. The toxicity and radioprotective activity of the compounds were studied on mice. The effect of prophylactic administration of these compounds on the hemopoietic system of irradiated animals was also studied. Values are given for LD/sub 16/, LD/sub 50/, and LD/sub 84/ used in the determination of toxicity of the preparations, and in studying the antiradiation activity, the mice were subjected to /sup 60/Co gamma-rays.