Low molecular weight natural mediators, 2'-5'-linked oligoadenylates, play an important role in interferon-based antiviral mechanism; participate in growth, apoptosis and other important cellular processes. The aim of current study was to find the evidence for the cell interaction with human apo-S100A1 using the methods of circular dichroism (CD) and heteronuclear NMR spectroscopy. Taking into account their concentration within living cells, the 2'-5'A3 oligoadenylates may act as additional biologically active substrates, capable of regulating the S100A1 protein functioning in vivo. The obtained results demonstrated the occurrence of the secondary structure changes in human S100A1 protein upon the interaction with 2'-5'-linked oligoadenylates as well as indicated specific residues involved in this process. Our study points to the 2'-5'-linked oligoadenylates as possible additional elements of the complex system of fine regulation of the Ca²⁺-signal transduction pathway in human cells.