Adrenochrome monoaminoguanidine (S-Adchnon) is a hemostatic capillary-stabilizing agent demonstrating pharmacological effects against radiation injury by reducing side effects of radiation therapy on hematopoietic organ. Synthesized by a dehydrating reaction of adrenochrome and aminoguanidine it has superior properties than adrenochrome, an oxidation product of adrenalin remarkable for its efficiency as a haemostatic agent at very small doses and for its more rapid and equally intense action than that of adrenalin. Adrenochrome does not alter the cardiac rhythm and does not cause any hypertension or internal haemorrhages and would be suitable for therapeutic applications, however, its instability, in aqueous or alcoholic solution, makes its use substantially impossible. S-Adchnon was devised, approved by the Japanese Ministry of Health, Labor and Welfare in 1962 and used widely in Japan. Adrenochrome monoaminoguanidine has negligible toxicity, stable and could be made into salts for aqueous dosage, especially for injection. Adrenochrome monoaminoguanidine methanesulfonate (AMM) enhances the recovery from radiation-induced leukopenia in rabbits and in humans, and inhibits the increases in chromosome aberrations in peripheral lymphocytes of patients with cervical carcinoma under radiotherapy. It has been shown that the radiation-induced initial decrease in number of peripheral blood leukocytes (PBL) is not affected by AMM, but recovery from the decrease is enhanced, shortening the period of leukopenia. This suggests that AMM may not exert its effects by protecting PBL directly but by protecting stem and/or progenitor cells in hematogenesis which proliferate and differentiate to PBL after irradiation. In in vitro colony formation method AMM demonstrated a protective effect on the survival of GM-CFC, a hematopoietic progenitor cells. Differential action on cancer and normal tissue by AMM and cytochrome C combined with radiotherapy was demonstrated. AMM in combination with cytochrome C augumented natural killer (NK) cells activity in KSN nude mice, protected potent NK cells in patients with lung cancer against radiotherapy and sensitized the human lung cancer xenografts to radiotherapy. Thus, AMM and cytochrome C may have the potential as a differential modulator of radiosensitivity of normal tissues and of tumors.