Fluorescence spectroscopy is a routine tool for studying cellular and molecular visualization, flow cytometry, medical diagnosis, forensic medical examination, genetic analysis, and also an analytical method. However, most available organic dyes used to produce an optical signal have a number of important limitations, such as hydrophobicity, low quantum yield, and low photostability. Further development of the luminescent method associated with the improvement of existing fluorophores is not always effective, so in recent years researchers have turned to the use of various nanomaterials that are used as nanoreactors or participants in the donor electron acceptor pair of electron excitation energy. Of particular interest are metallic nanoparticles that can increase or decrease the fluorescence lifetime of fluorophores, increase the efficiency of resonant energy transfer in the donor-acceptor system as a result of the interaction of the excited state of a fluo- rophore with free electrons in metal nanoparticles. In the present work, the influence of silver nanoparticles on the fluorescent properties of doxycycline (DC) and its complex with europium is considered. The nanoparticles obtained are spherical, a narrow range of size distribution with an average diameter of 20 ± 1.5 nm, and a ?-potential of -30 mV. The stability of the characteristics is maintained for 14 days. In order to reduce the distance between the metal surface and the DC molecule, the silver nanoparticles are modified by europium ions. Earlier we showed that the DC forms a chelate with europium ions, which is characterized by the transfer of excitation energy. A peculiarity of complexation is the appearance in the fluorescence spectrum of a new band, which is characteristic for the emission of the europium ion (?fl = 615 nm). We have established, that the ?-potential of modified nanoparticles depends on the concentration of europium ions in the solution. In the presence of europium ions, the ?-potential of the nanoparticles increases from -30 mV to zero (4·10-4M Eu3+) and under the conditions of further growth of the metal ion concentration (up to 1.25 ·10-3M) the potential value increases (+85 mV), which causes a highstability of the colloidal system. We have established, that the signal of sensitized fluorescence in the presence of modified nanoparticles depends on the acidity of the medium and the concentration of silver nanoparticles. Optimal conditions for obtaining the maximum analytical signal are found. Based on the studies carried out, a fluorimetric method for determi- nation of DC in the drug “Doxycycline”, LLC “Ozone” was developed.