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Aim: Ceria nanoparticles (NPs) are potent anti-oxidants because of their small size and surface characteristics. In addition to this, they have anti-inflammatory effects. However, their effects on post-stroke inflammation have not been evaluated in experimental intracerebral hemorrhage (ICH).Methods: Ceria NPs were made by a modified double-decomposition method. RAW 264.7 macrophages (MΦs) were activated by 30% hemolysate from blood by checking morphology and inflammatory products. They were treated by 0.125 mM ceria NPs or not. Overall reactive oxygen species (ROS) after the activation of MΦs were measured by oxidized DCF-DA in FACS. Inflammatory cytokines, IL-1β and TNF-α, were evaluated by ELISA, and inflammatory end-products, COX2 and iNOS, were done by Western blots. Animal ICH was induced by the stereotactic injection of collagenase into the rat brain, and ceria NPs were injected intravenously. Water contents were measured by wet and dry weights of brain after induction of ICH. We calculated hematoma volumes by an image-analyzer and the recruitment of MΦs in perihematomal area by the immunofluorescent staining using CD68 antibody.Results: Ceria NPs were made as 3-nm small sized, uniform round-shaped, and well-dispersed agents, which were suitable for biomedical applications. Ceria NPs reduced oxidized DCF-DA (a marker of overall ROS) after hemolysate-induced activation of MΦs (ceria-treated group, 391 unit vs. control, 567 unit, P=0.03). In this in vitro hemorrhage model, MΦs secreted less cytokines, IL-1β and TNF-α, in ceria-treated group than control, and in Western blots, the expression of COX2 and iNOS decreased in ceria group compared to control. In rat ICH model, ceria NPs reduced water contents in hemorrhagic hemisphere, which is closely related to perihematomal inflammation (0.5 mg/kg ceria group, 79,9% vs. control, 81.1%, P=0.01). Although hematoma volumes were not different between two groups, the CD68-positive MΦs in immunofluorescent staining were less recruited into the perihematomal area in ceria-treated group than control.Conclusion: Ceria NPs decreased inflammation after ICH in both in vitro and in vivo models. From these anti-inflammatory effects, they could be potential therapeutic agents in acute period after ICH.