The biological activities of promegestone (R 5020), used world-wide as a radioligand for the progestin receptor, are described. Promegestone is a potent progestin devoid of androgenic side-effects and has marked anti-estrogenic activity. Its therapeutical effectiveness (0.125 mg or 0.250 mg) in luteal insufficiency has been established in a double-blind clinical trial versus dydrogesterone (10 or 20 mg) which revealed its significantly superior potency (p less than 0.001). On the basis of the pharmacological data described, its use in cancer therapy according to a sequential protocol for the treatment of hormone-dependent breast cancer and in psychiatry to palliate the psychological disorders of the premenstrual syndrome and the perimenopause seems promising.