Quantum dots have been delivered directly across the plasma membrane to the cytosol of living cells using a combination of a cationic peptide, polyarginine, and a hydrophobic counterion, pyrenebutyrate. Quantum dot delivery did not disrupt the plasma membrane and bypassed the barrier of endocytic vesicles. Cellular uptake was independent of temperature but highly dependent oil the surface charge of the quantum dot and the membrane potential of the cell, suggesting a direct translocation across the membrane. This method of delivery can find immediate application for quantum dots and may be broadly applicable to other nanoparticles.