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Industrial application of titanium dioxide nanoparticles (TiO2-NPs) as an additive in pharmaceutical and cosmetic products is increasing. However, the knowledge about the toxicity of this material is still incomplete and data concerning health and environmental safety and results of recent studies on TiO2 nanotoxicology are inconsistent. The in vitro geno- and cytotoxicity of TiO2-NPs in the anatase crystal phase was evaluated in human peripheral blood lymphocytes from 10 male donors. Initially, transmission electron microscopy (TEM) was performed to describe particle morphology and size, the degree of particle aggregation, and the intracellular distribution. Cells were exposed to nanoparticles in increasing concentrations of 20, 50, 100, and 200 μg/ml for 24 hr. Cytotoxic effects were analyzed by trypan blue exclusion test and the single-cell microgel electrophoresis (comet) assay was applied to detect DNA double-strand breakage. TiO2-NPs were sphere shaped with a diameter of 15–30 nm. Despite dispersive pretreatment, a strong tendency to form aggregates was observed. Particles were detected in the cytoplasm of lymphocytes, but also a transfer into the nucleus was seen. The trypan blue exclusion test did not show any decrease in lymphocyte viability, and there was no evidence of genotoxicity in the comet assay for any of the tested concentrations. In conclusion, TiO2-NPs reached the cytoplasm as well as the nucleus and did not induce cyto- or genotoxic effects in human peripheral blood lymphocytes. Complement investigations on different human cell systems will be performed to estimate the biocompatibility of TiO2-NPs. Environ. Mol. Mutagen., 2011. © 2010 Wiley-Liss, Inc.