The cytotoxicity and cellular internalization of silica nanotubes (SNTs) of varying sizes and surface charges were evaluated against a panel of cell lines in vitro. A series of SNTs of 50 nm diameter and 200 nm or 500 nm length were template-synthesized. Bare SNTs of each length were further modified by (3-(aminopropyl) triethoxysilane (APTES) on the outer surface. The inner surface of SNTs was modified with Alexa fluor 488 molecules. The SNTs characterization was carried out with TEM and Malvern Instrument Zetasizer Nano (Westborough, MA). Cytotoxicity of SNTs was assessed by MTT and LDH assays. Cellular uptake of SNTs was visualized by laser scanning confocal microscope Olympus FV1000. Based on the results of MTT assay, no significant effects on cell proliferation were observed for all SNTs in the range of used concentrations. The 500 nm bare SNTs showed no effect on cell growth for all tested cell lines. Surface modification resulted in slightly higher toxicity of positively charged SNTs. The 200 nm A+ and 500 nm A+ SNTs inhibited proliferation of WI-38 cell resulting decrease of cell population by 40-45% compared to untreated cells. 200 nm A+ SNTs appeared to be slightly more toxic to other cell lines. However, no toxicity higher than 45% was observed even at a concentration 5 ï­g/ml. It appears that toxic effect of 200 nm A+ SNTs was cell type dependent, with human lung fibroblasts being more sensitive to the treatment. Further evaluation of 200 nm A+ SNTs toxicity showed that these SNTs do not compromise the integrity of MCF 10A cell plasma membrane, indicating that toxicity of the SNTs on these cell lines at the concentrations studied mostly depend on their interaction with intracellular components. Fluorescent confocal microscopy revealed that Alexa fluor 488 labeled SNTs were associated with H460 cells and were located inside the cells. The fluorescent SNTs primarily were localized in perinuclear regions, but not in the nucleus of the cells. It appears that cells with 200 nm A+ have brighter fluorescence compared to negatively charged counterparts. Some cells had no green fluorescence demonstrating uneven exposure of cells to the SNTs. Our results indicate that SNTs can be internalized by cultured cells at the size and surface charge range studied. SNTs have low toxicity toward cultured cells. Smaller size and positive surface charge make SNTs more cytotoxic.