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DNA cages are nanometer-scale polyhedral structures formed by self-assembly from synthetic DNA oligonucleotides. Potential applications include in vivo imaging and the targeted delivery of macromolecules into living cells. We report an investigation of the ability of a model cage, a DNA tetrahedron, to enter live cultured mammalian cells. Cultured human embryonic kidney cells were treated with a range of fluorescently labeled DNA tetrahedra and subsequently examined using confocal microscopy and flow cytometry. Substantial uptake of tetrahedra into cells was observed both when the cells were treated with tetrahedra alone and when the cells were treated with a mixture of tetrahedra and a transfection reagent. Analysis of the subcellular localization of transfected tetrahedra using confocal microscopy and organelle staining indicates that the cages are located in the cytoplasm. FRET experiments indicate that the DNA cages remain substantially intact within the cells for at least 48 h after transfection. This is a first step toward the use of engineered DNA nanostructures to deliver and control the activity of cargoes within cells.