Human lactoferrin exhibits many unique properties. It is known as one of the most important factors that provide nonspecific defense of cells against bacteria, viruses, and carcinogenesis, as well as an important component of a specific system responsible for the passive immunity of newborns. As a compound with extremely broad spectrum of functions many of which were not elucidated so far, lactoferrin is intensely studied. In this study we obtained electrophoretically and immunologically homogenous preparations of lactoferrin from human milk. Using various methods, we were the first to show that the fraction of lactoferrin, which displays an increased affinity for Sepharose Blue, forms complexes with ATP with a stoichiometry of 1 mole ATP per mole protein. It is shown that the ATP-binding site is located in the C-terminal domain of the lactoferrin molecule. The binding of ATP results in the dissociation of tetrameric forms of the protein and a change in the mode of interaction of lactoferrin with polysaccharides and other proteins. The data may be used in analysis of the possible reasons for multifunctional properties of lactoferrin and possible ways of regulation of its functions.