Exposure to fetal bovine serum (FBS) is shown herein to reduce the aggregate size of titanium dioxide (TiO(2)) nanoparticles, affecting uptake and consequent effect on A549 and H1299 human lung cell lines. Initially, the cellular uptake of the FBS-treated TiO(2) was lower than that of non-FBS-treated TiO(2). Expulsion of particles was then observed, followed by a second phase of uptake of FBS-treated TiO(2), resulting in an increase in the cellular content of FBS-treated TiO(2), eventually exceeding the amount by cells exposed to non-FBS-treated TiO(2). Surface adsorbed vitronectin and the clathrin-mediated endocytosis pathway were shown to regulate the uptake of TiO(2) into A549 cells, while the endocytosis mechanism responsible remains elusive for H1299. Intriguingly, nystatin treatment was shown to have the unexpected effect of increasing nanoparticle uptake into the A549 cells via an alternate endocytic pathway. The surface adsorbed serum components were found to provide some protection from the cytotoxic effect of endocytosed TiO(2) nanoparticles.