Data are given on lymphokins, the biologically active substances lymphocytes when the latter are stimulated with specific antigens or unspecific mitogens. Discussed are their essence, antigen specificity, cell origin, and the methods of purification, standardization, and evaluation of biologic importance. It is believed that the lymphokins appear to be the products of primary activation of T lymphocytes. They possess pharmacologic activity, but have no antigenic specificity. Their function consists in mediating hypersensitive and inflammatory reactions through the activation of phagocytes. The effectiveness of cellular immunity depends on the activity of these factors.