The cytotoxicity potential of TiO(2) and Al(2)O(3) nanoparticles (NP) in UMR 106 cells was studied by evaluating the lysosomal activity with neutral red uptake assay (NR), and the mitochondrial activity with tetrazolium MTT test. Different NP concentrations (10-300 microg/mL range) were used. A significant (p < 0.001) increase in the absorbance (stronger for TiO(2) NP) was detected in both NR and MTT assays after 24-h exposure to the NP. However, the total cell proteins and the cell proliferation rate demonstrated (p < 0.05) that the cell viability decreased after 96 h exposure to NP. The formation of NP-containing vesicles within the cells was observed by transmission electronic microscopy. Such event could explain the high cellular activity detected during the early stages of exposure not related to the increase in cell viability. Results showed that the effects of NP on cell lines are dependent on the chemical composition of the particles, their concentration, exposure time, and the type of treated cell. It can be concluded that the presence of TiO(2) and Al(2)O(3) NP in the cell surroundings can lead to cytotoxic effects. In the case of osteoblast cells, such events may induce osseointegration failures in orthopedic and dental implants that release NP.