The aim of the work was the synthesis of gold nanoparticles (GNP) of different sizes and the estimation of their biological activity in vitro and in vivo. Materials and Methods: Water dispersions of gold nanoparticles of different sizes have been synthesized by Davis method and characterized by laser-correlation spectroscopy and transmission electron microscopy methods. The GNP interaction with tumor cells has been visualized by confocal microscopy method. The enzyme activity was determined by standard biochemical methods. GNP distribution and content in organs and tissues have been determined via atomic-absorption spectrometry method; genotoxic influence has been estimated by “Comet-assay” method. Results: The GNP size-dependent accumulation in cultured U937 tumor cells and their ability to modulate U937 cell membrane Na+,K+-АТР-ase activity value has been revealed in vitro. Using in vivo model of Guerin carcinoma it has been shown that GNP possess high affinity to tumor cells. Conclusions: Our results indicate the perspectives of use of the synthesized GNP water dispersions for cancer diagnostics and treatment. It’s necessary to take into account a size-dependent biosafety level of nanoparticles.