The effects of silicon dioxide-based nanoparticles on the viability and proliferative activity of human peripheral blood cultured lymphocytes were studied. All nanoparticles in a concentration of 100 μg/ml produced a significant cytotoxic effect, its intensity depending on particles' structure: SiO2 nanoparticles were least toxic, while Ce3(+)-intercaled montmorillonite nanoparticles were most toxic. The cells died mainly by apoptosis and postapoptotic necrosis. Incubation with nanoparticles in a concentration of 100 μg/ml for 72 h caused death of all phytohemagglutinin-activated lymphocytes, while in concentrations of 1 and 10 μg/ml the nanoparticles had no effect of proliferative activity of cells. The results suggest that the effects of nanoparticles on cells are determined by the nanoparticle concentration and size, as well as by their structure.