Cellulose nanocrystals are promising candidates for applications in nanomedicine. To enable the use of fluorescence techniques in in vitro and in vivo studies, cellulose nanocrystals were labeled with fluorescein-5'-isothiocyanate (FITC) via a three-step reaction, involving epoxy activation of the nanocrystal surface, opening of the epoxy rings with ammonium hydroxide, and coupling of FITC molecules to the primary amino groups. The FITC content of the labeled cellulose nanocrystals was determined by UV/vis spectroscopy. A FITC content of 0.03 mmol/g of cellulose, equivalent to 5 FITC moieties per 1000 anhydroglucose units, was obtained using the described method.