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The formulation of plasmid DNA on 100 nm gold nanoparticles surface-tethered via cationic dendrons, and the behaviour of the complex in cell culture media, is described in this communication. Adsorption of dendrons onto gold nanoparticles in water resulted in the generation of positively charged nanoparticles with a corresponding small increase in particle size. Addition of plasmid DNA did not markedly reduce the surface potential but resulted in a ∼10–20% increase in hydrodynamic diameter. More dramatic effects were seen in the presence of cell culture media that, overall, drastically increased the apparent size of the gold–dendron–DNA nanoparticles and reduced the surface potential of the colloids, the presence of serum components partially ameliorating these effects possibly due to steric stabilisation. Release of the surface-tethered DNA was reduced in cell culture media compared to water. This reduced detachment of DNA coupled with the flocculation of the carrier which would likely inhibit endocytosis, demonstrates the importance of testing drug delivery systems with relevant physiologically based fluids prior to their use in vivo studies.