The nuclear delivery of nucleic acid derivatives is an essential prerequisite for successful antisense therapy. Using laser confocal and electron microscopy, we have studied the uptake of fluorescently labeled oligonucleotides in the form of nanocomposites with polylysine and TiO₂ nanoparticles into Caco2, MDCK, and HeLa cells. In all three cell lines, bright fluorescence has been detected after 30 min in the nuclei (excluding the nucleoli) of the interphase cells; no substantial increase in the intensity of the signal was observed for next 24 hours. In all cells undergoing mitosis, the signal was localized in the cytoplasm with zones of higher intensity around chromatin. In some cells, at the beginning of interphase (G-1 phase), fluorescence was not detected at all. The latter may be explained by the brief moment in the cell cycle when oligonucleotides delivered in the nanocomposite cannot be taken up by cells. The studied nanocomposites are prone to aggregation. The degree of aggregation increases with the storage time up to complete loss of the ability of the nanocomposites to penetrate the cells.