Hydroxyapatite (Ca10(PO4� 6(OH)2, HA) nanoparticles activated with Eu 3+ were synthesized using the sol–gel technique. The crystal structure was elucidated by X-ray diffraction analysis and the internal crystal structure and morphology was studied by transmission electron microscopy. Under excitation with 394 nm light, the powders are red-emitting. The photoluminescence bands are detected at 575 nm ( 5 D0 → 7 F0), ∼590 nm ( 5 D0 → 7 F1), 613 nm ( 5 D0 → 7 F2) and 626 nm ( 5 D0 → 7 F2). The nanosized HA:Eu dispersed in urine and blood serum showed high stability with respect to their optical properties, although some transitions underwent a change in relative emission intensities. A change in emission intensities was also observed for the cysteine dispersion. The stability at different pHs was studied in water and urine fluids. The HA:Eu nanoparticles are suitable as biomarkers for nanomedical applications.