Abstract <p>A protective effect of CeO₂ nanoparticles on human serum albumin (HSA) under nitrosative stress induced by peroxynitrite (ONOO^–) was demonstrated. The effect of nanodisperse CeO₂ and ONOO^– on the tryptophan fluorescence of purified HSA, as well as albumin isolated from biological fluids, blood plasma, peritoneal (ascitic), and synovial (joint) fluids, was examined for the first time by fluorescence spectroscopy. ONOO^– decreased the intrinsic fluorescence of HSA, whereas preliminary incubation of the protein with a CeO₂ sol markedly attenuated this effect. In the presence of nanodisperse CeO₂, the degree of oxidative modification of albumin induced by ONOO^– decreased by ~20%. The results further indicate the substantial potential of nanodisperse CeO₂ for biomedical applications.</p>