β-Mercaptoethylguanidine Br·HBr (MEG) was less effective than cysteamine (MEA) in protecting HeLa S3 cells against X-irradiation. Protection by MEG developed gradually with time of incubation, whereas the action of MEA was very rapid. Low temperature reduced the development of protection in the cases of MEA and MEG, but to a larger extent in the latter case. Washing of the MEA-treated cells resulted in a rapid decay of the protective potency. On the other hand, MEG-treated cells remained similarly protected for 15 minutes after removal of MEG from the culture medium. N,N′-Dimethyl derivative of MEG was less protective than MEG. Prolonged incubation did not improve the activity of the dimethyl-MEG. Experiment with the use of14 C-labeled MEG showed that the time course of development of protection closely resembled the time course of radioactivity incorporation into the cells. After the cells were incubated with <tex-math>${}^{14}{\rm C}\text{-}{\rm MEG}$</tex-math>, distribution of radioactivity among subcellular particles was examined. Approximately one third of the radioactivity was found in the nuclear fraction. A large portion of this nuclear radioactivity was not extractable with acid.

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