[en] The authors investigated the protective effects of WR2721 on the induction by radiation of altered hepatocyte foci in 150-day-old Sprague Dawley rats. One hundred micrograms of WR2721 per gram of body weight were administered to selected groups of neonatal animals 30 min prior to the administration of single doses of γ irradiation (150 or 300 rad). Focus pheotypes, gamma-glutamyltranspeptidase (GGT) and/or the iron exclusion histochemical markers, were quantitated through the use of serial frozen sectioning techniques and computer-assisted image analysis. Radiation was effective in inducing foci formation. Induction was much more effective in female as compared to male rats. WR2721 reduced the frequencies of radiation-induced foci at both radiation doses, with the greatest effect apparent in female rats. The protective effect of WR2721 in this system suggests that this compound and related aminothiols may be useful probes for examining mechanisms of mutagenesis and carcinogenesis induced by radiation or chemicals