Engineered amorphous silica nanoparticles (SiO(2)-NPs) are widely used in dyes, varnishes, plastics and glue, as well as in pharmaceuticals, cosmetics and food. Novel composite SiO(2)-NPs are promising multifunctional devices and combine labels for subsequent tracking and are functionalized e.g. to specifically target cells to deliver their cargo. However, biological and potential toxic effects of SiO(2)-NPs are insufficiently understood. The aim of this study was to determine the uptake and fate of SiO(2)-NPs in mammalian cells. Also, silica submicron particles (SiO(2)-SMPs) were included in the studies in order to identify effects, which are only observed for nano-sized SiO(2) particles. Fluorescently labelled SiO(2)-NPs (nominal size 70 nm) and SiO(2)-SMPs (nominal size 200 and 500 nm) were used to examine cytotoxicity, cellular uptake and localization in human cervical carcinoma cells (HeLa). Particle uptake and intracellular localization in mitochondria, endosomes, lysosomes and nuclei were studied by wide field and confocal laser scanning fluorescence microscopy. Physicochemical characterization of SiO(2)-NPs by transmission electron microscopy and dynamic light scattering revealed a spherical morphology and a monodisperse size distribution. In the presence of serum, all SiO(2) particles are non-toxic. However, in the absence of serum SiO(2)-NPs but not SiO(2)-SMPs are highly toxic. SiO(2) particles, irrespective of size, were detected in the cytosol and accumulated in endosomal compartments of HeLa cells. No accumulation of SiO(2) particles in nuclei or mitochondria of HeLa cells could be observed. In contrast to SiO(2)-SMPs, SiO(2)-NPs are preferentially localized in lysosomes.