Leonid Vasilyevich Sobolev is an outstanding pathologist, whose name definitely should occupy an honorable place in the galaxy of great scientists of Russia. However, his name was undeservedly forgotten, and the role of his work was underestimated. This Russian scientist made the most important discovery: he proved in an experiment that it is the islets of Langerhans in the pancreas that secrete a humoral regulator, "factor X", the deficiency of which leads to diabetes mellitus. This mysterious islet factor will be isolated from the pancreas of dogs in 1921 by the future Nobel laureate Frederick G. Banting and will become part of the medicine as insulin. However, there is every reason to believe that the discovery of F. G. Banting, who repeated a series of experiments by Leonid Vasilyevich Sobolev, is, in fact, secondary, which is the research subject in this paper.