As silica nanoparticles (SiO2 NP) gain increasing interest for medical applications it is important to understand their potential adverse effects for humans. Here we prepared well-defined core-shell fluorescently labelled SiO2 NP of 15, 60 and 200 nm diameter and analyzed their cytotoxicity in THP-1 derived macrophages, A549 epithelial cells, HaCaT keratinocytes and NRK-52E kidney cells. We observed a size-dependent cytotoxicity in all cell types in serumfree conditions. HaCaT cells were least and macrophages or lung derived A549 cells were highly sensitive towards SiO2 NP treatment. Differences in cytotoxicity could be correlated with different uptake rates. By using flow cytometry and confocal microscopy we quantified the uptake. Furthermore we used specific inhibitors for clathrin- and caveolinmediated endocytosis to elucidate the uptake mechanisms, which were found to be dependent on the NP size and the cell type. Clathrin-mediated endocytosis was involved in the uptake of SiO2 NP of all sizes and was the major pathway for 60 nm or 200 nm SiO2 NP. Caveolin-mediated endocytosis contributed to the uptake of 60 and 200 nm SiO2 NP in THP-1 macrophages but only to uptake of 200 nm SiO2 NP in A549. However, in the presence of serum all SiO2 NP were non-toxic. The presence of serum furthermore could alter the uptake mechanism. In summary, this study demonstrates size- and cell type dependent differences in SiO2 NP uptake and toxicity.