Fluorescently labeled cellulose nanofibrils (CNFs) were used to evaluate CNF leaching from paper according to standard safety assays for food contact materials. Enzymatically pretreated pulp was first labeled with 5-([4,6-Dichlorotriazin-2-yl]amino)fluorescein hydrochloride (DTAF), followed by homogenization to produce fluorescent CNFs of varying degrees of fibrillation. Labeling at the mu molar DTAF/g cellulose level imparted quantitative ppb fluorescence detection of CNFs (LOD of approximately 20 ppb), without significantly altering other material properties, suggesting that DTAF-labeled CNFs are an appropriate mimic for native CNFs and that this approach can be used to detect low CNF concentrations. Cold and hot-water extractions of laboratory papers (100 % CNFs and CNF-fiber blended papers) showed loss values below 3 wt% CNFs, with the finest CNF quality showing the least loss overall and with greater loss experienced under hot water conditions compared with cold water. DTAFlabeled CNFs can be used to address questions related to CNF distribution, localization, and loss.