Abstract: P-aminoazobenzene hydrochloride showed a marked radioprotective effect in mice irradiated with lethal doses of 60Co γ-rays.The effective dose of the drug was 50 mg/kg body weight which was administered either intraperitoneally or per os before irradiation.The advantages of this drug are:Both oral and intraperitoneal routes of administration are effective,the latency of appearance of protective effect is relatively short and the range of effective doses is wide.Furthermore,the chemical properties of this drug is stable under room temperature.On the other hand,the formation of metahemoglobin may be considered as one of the drawbacks of this drug,but within the range of effective doses it is well tolerated by the animals.The mechanism of protection of this drug has been discussed based on our preliminary work elucidating this point.