AbstractNanodiamonds that were prepared by high pressure/high temperature were functionalised with biomolecules for biological applications. Nanodiamonds (NDs, ≤35 nm) that were coated by silanisation or with polyelectrolyte layers were grafted with a fluorescent thiolated peptide via a maleimido function; this led to an aqueous colloidal suspension that was stable for months. These substituted NDs were not cytotoxic for CHO cells. Their capacity to enter mammalian cells, and their localisation inside were ascertained after labelling the nucleus and actin, by examining the cells by confocal, reflected light and fluorescence microscopy.