We investigated the toxicity of aggregated nanoparticles of cerium oxide (CeO 2 ) using an in vitro 3D human bronchial epithelial model that included a mucociliary apparatus (MucilAir™). CeO 2 was dispersed in saline and applied to the apical surface of the model. CeO 2 did not induce distinct effects in the model, whereas it did in BEAS-2B and A549 cell cultures. The absence of effects of CeO 2 was not because of the model’s insensitivity. Nanoparticles of zinc oxide (ZnO) elicited positive responses in the toxicological assays. Respiratory mucus (0.1% and 1%) added to dispersions increased aggregation/agglomeration to such an extent that most CeO 2 sedimented within a few minutes. Also, the mucociliary apparatus of the model removed CeO 2 from the central part of the apical surface to the borders. This ‘clearance’ may have prevented the majority of CeO 2 from reaching the epithelial cells. Chemical analysis of cerium in the basal tissue culture medium showed only minimal translocation of cerium across the 3D barrier. In conclusion, mucociliary defence appeared to prevent CeO 2 reaching the respiratory epithelial cells in this 3D in vitro model. This model and approach can be used to study compounds of specific toxicological concern in airway defence mechanisms in vitro.