[en] Protection against X-irradiation by a number of cysteamine derivatives was studied in tissue culture and the results were compared with data obtained in mice. Compounds with a covered SH group like WR 638, cysteamine phosphate, WR 2721 and AE 48527, showed practically no protection when dissolved in tissue culture medium, but developed a protective activity when dissolved in rat blood. Thiol measurements demonstrated that in rat blood the compounds were partly hydrolysed to thiols. C511 was also hydrolysed in culture medium and was slightly less effective than cysteamine in culture medium. Cysteamine phosphate was hydrolysed more easily than cysteamine sulphate and concordantly the protective activity in rat blood was better. WR 2721 was also partly hydrolysed in rat blood. The in vitro protection of this compound was disappointing when compared with results in vivo. Its SH form (WR 1065) also showed less protection than expected from in vivo experiments. Thus the little protection by WR 2721 in vitro in rat blood is not only due to its incomplete conversion into its thiol. Longer incubation times and the use of rat blood as a solvent brought the protective activity of WR 1065 up to almost the level of cysteamine. This may indicate that WR 1065 is poorly penetrating into the cells. WR 1065 was the only compound studied whose protective activity in vitro was improved appreciably by dissolving it in rat plasma