The protective effect of zinc against radiation hazard was observed by comparing LD50(30) values of irradiated mice with and without the administration of zinc before irradiation. Two hundred male mice, 7 weeks old, were irradiated with an intestinal dose at 5.62, 6.31, 7.08, 7.94, 8.91, or 10.00 Gy, with or without oral administration of zinc from 10 days prior to gamma irradiation until the end of the study. The reduction of mortalities in zinc-administered mice in comparison to controls was most prominent at the dose of 7.94 Gy, i.e., the mortality rate in the zinc-administered group was 0.44 and that in the control group was 1.00. At a dose of 8.91 Gy, zinc-administered mice survived significantly longer than controls. The LD50(30) of zinc-administered mice was 7.70 Gy, while that of the control was 6.90 Gy. In zinc-administered mice the acceleration of turnover and elevated excretion of radioactive zinc were observed, while the concentration of stable zinc in organs remained constant. The effect of zinc was not observed in experiments in vitro using human melanoma cells. Therefore it can be assumed that the protective effect of zinc against radiation would correlate with the homeostatic mechanism of zinc which exists in the whole body of mammalians.