The activity of the nano- and microparticles of titanium dioxide (TD) given in doses of 1.6, 40.0, and 500 microg/ml was studied in the cytochalasin B block micronucleus test with blood cells from two donors. It was shown that nano-DT in doses of 1.6 and 40 microg/ml was more active than micro-DT in inducing micronuclei and nucleoplasmic bridges, asymmetric cell division, and accelerated cell proliferation. Some differences in individual sensitivity to TD between donor blood cultures were due to an inverse trend in the association between the indicators of genomic damage and apoptosis. The new nuclear morphometry test that had developed to screen genotoxic effects and used in this study for the first time revealed a nano-DT dose-depended increase in the asymmetry of nuclei in the binucleated cells in the cultures of both blood donors, the morphometric effects being directly correlated with the values of cytogenetic analysis.