Abstract: Derivatives of imidazole, e. g., pilocarpine, histidine and the naturally occurring purine derivatives usually possess notable physiological activity. As reported by Arbusov, 8-(β-aminoethylmercapto) caffeine has a marked protective effect against ionizing radiation on mammals. This paper describes the synthesis of a number of sulphur-containing imidazole derivatives for the study of its biological action. 8-(β-aminoethylmercapto) caffeine (Ia)and 8-(tetraacetylglucosylmercapto) caffeinc (Id) are prepared by the reaction of 8-mercapto-caffeine with the corresponding bromocompounds. Ia may be converted to 8-(β-guanidoethylmercapto) caffeine (Ib) with S-me-thylthiourea in the usual manner. By the reaction of 8-bromocaffeine with cysteine followed by esterification, 8-(β-amino-β-carbomethoxyethylmecapto) caffeine (Ic) results. On the other hand, 8-bromocaffeine, when treated with β-hydroxyethylamine, gives 8-(β-hydroxyethylamino) caffeine (Ic). The latter, after conversion into 8-(β-S-isothiuronium-ethylamino) caffeine (Ig), gives 8-(β-mercaptoethylamino) caffeine (Ib) on hydrolysis. Ih and Ig may be regarded as the structural analogues of Ia and Ib respectively. Preliminary biological tests indicate that these sulphur-containing derivatives of caffeine have conspicuous protective effect against ionizing radiation on experimental rats. For the purpose of studying whether the purine skeleton is essential, or the protective action is associated with the central nerve stimulating effects, sulphur-containing derivatives of imidazole but not of purine have also been prepared. These include those of benzoimidazole and of imidazole itself. These are known to have no stimulating effect on the central nerve system. The derivatives of imidazole itself may also be regarded as the cyclized analogue of β-aminoethylthiuronium bromide hydrobromide (AET), the well known chemical protective agent against radiation By refluxing the 2-mercaptocompounds of benzoimidazole, 4-methylimidazole, 4-phenylimidazole as well as imidazoline with β-bromoethylamine hydrobromide in ethanol, the sulphydryl hydrogen was replaced, giving the corresponding 2-(β-aminoethylmecapto)-derivatives (Ⅲa, b, c). These compounds give no colour reaction with sodium nitroprusside, indicating absence of the free mercapto group. They form well crystallizing salts with mineral acids. The IR-spectra of these compounds show maxima on 660—640 cm^-1 characteristic for C-S-C linkage. The biological activities of these compounds will be published elsewhere.