Evidence is given in support of the hypothesis that the radioprotective action of a group of sulfhydryl compounds such as cysteine and cysteamine is not based on the removal of oxygen but is caused by another mode of action on the cellular level. In the experiments, cysteamine in concentrations of 4 and 16 mM- bar gave dose reduction factors of 1.8 and 3.1, respectively. Under anoxic conditions where no oxygen could be demonstrated a dose reductlon factor of 2.5 was obtained. Since 16 mM-bar cysteamine gave a higher dose reduction factor than complete anoxia, the protective action of cysteamine cannot be explained as an effect caused by anoxia only. This was also shown by the fact that 16 mM-bar cysteamine under anoxic conditions gives better protection than 16 mM-bar cysteamine only or an anoxic condition only. A third demonstration that the radioprotective action of cysteamine is not caused by an anoxia induced by its autoxidation was found in an experiment in which air instead of nitrogen was flushed over cells treated with cysteamine. (P.C.H.)