The main line of scientific research and main practical achievements of L. A. Ilyin can be represented as follows: the establishment of the first Black Sea Fleet nucleonics laboratory, development of new drugs and personal protection means from the effects of gamma-neutron radiation, incorporation of radionuclides in the body and contact radioactive contamination of skin, wounds and burns; the development of the issues of medical and hygienic protection of personnel and population in the process of establishment and development of new nuclear technologies and in case of radiation accidents; regulation of permissible levels of human exposure; radiobiology of the low dose rate exposure and stochastic effects of human exposure forecast; the development of the practical threshold dose concept in radiation epidemiology and hygienic standards. He repeatedly took part, sometimes as a scientific supervisor, in the new drug trials on landfills. For the first time in the world L. A. Ilyin elaborated the understanding of the radiological consequences of the Chernobyl accident, later confirmed by the leading international and Russian experts.