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Aggregation of metal oxide nanoparticles in aqueous media complicates interpretation of in vitro studies of nanoparticle–cell interactions. We used dynamic light scattering to investigate the aggregation dynamics of iron oxide and zinc oxide nanoparticles. Our results show that iron oxide particles aggregate more readily than zinc oxide particles. Pretreatment with serum stabilises iron oxide and zinc oxide nanoparticles against aggregation. Serum-treated iron oxide is stable only in pure water, while zinc oxide is stable in water or cell culture media. These findings, combined with zeta potential measurements and quantification of proteins adsorbed on particle surface, suggest that serum stabilisation of iron oxide particles occurs primarily through protein adsorption and resulting net surface charge. Zinc oxide stabilisation, however, also involves steric hindrance of particle aggregation. Fluid shear at levels used in …