The article covers the life and activities of Professor Arseniy Danylenko, senior researcher at the Laboratory of Biophysics of the O.O. Bogomolets Institute of Physiology. He stood at the origins of the research on ionizing radiation of very low intensities and methods of measuring them. For the first time, he determined the levels of natural radioactivity in different environments (atmosphere, soil, plants, animals, humans) and obtained important data on the role of microdoses of ionizing radiation in the life of an organism. Based on the study of the beta activity of the blood of healthy people and people suffering from various diseases, he established the limits of fluctuations in the level of activity of beta radiation, characteristic of these conditions. He was one of the initiators of the use of artificial radioactive isotopes in the diagnosis of tumors.