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Development of aptatheranostics for glioblastoma (GB) requires investigating aptamer interactions with cells. The paper has described flow cytometry (FC) assessment of direct interactions of fluorescent anti-CD133 aptamers with cells, focusing on cell cultures derived from patient GB (CCPGB). Conventional cell lines with different levels of CD133 mRNA, Caco-2 and HCT116, were used to compare interactions with known 2′FY-RNA aptamer A15 and DNA aptamers of Ap and Cs series, labeled with FAM and Cy5. In addition, interactions of certain non-aptameric oligonucleotides were studied. In the case of antibody interactions with cells, FC signals, mean fluorescence intensities (MFIs), correlated with sizable amounts of CD133 mRNA in Caco-2 cells, and CCPGBs 107 and G01. Unexpectedly, MFI per se could not be the solid indicator of specific interactions of aptamer - CD133/cell. Instead, two types of interactions, target CD133-driven and off-target membrane-associated ones, contribute to MFI. The latter was notably observed for CCPGB Sus/fP2 with tiny CD133 mRNA amount. To prove specificity of aptamer - CD133/cell interactions, titration experiments have been performed, revealing half-saturation concentrations of 120±27 for 2′FY-RNA A15 and 180±12 for DNA Cs5 with Caco-2 cells. This knowledge is an essential step to develop aptatheranostics for GB.