Fluorescent nanodiamond (FND) has excellent biocompatibility and photostability, making it well suited for long-term labeling and tracking of cancer and stem cells. To prove the concept, the exocytosis of FND particles (size ≈100 nm) from three cell lines--HeLa cervical cancer cells, 3T3-L1 pre-adipocytes, and 489-2.1 multipotent stromal cells--is studied in detail. FND labeling is performed by incubating the cells in a serum-free medium containing 80 μg mL(-1) FND for 4 h. No significant alteration in growth or proliferation of the FND-labeled cells, including the multipotent stromal cells, is observed for up to 8 days. Flow cytometric analysis, in combination with parallel cell doubling-time measurements, indicates that there is little (≈15% or less) excretion of the endocytosed FND particles after 6 days of labeling for both HeLa and 489-2.1 cells, but exocytosis occurs more readily (up to 30%) for 3T3-L1 preadipocytes. A comparative experiment with FND and the widely used dye, carboxyfluorescein diacetate succinimidyl ester, demonstrates that the nanoparticle platform is a promising alternate probe for long-term cell labeling and tracking applications.