The importance of nanotechnologies and engineered nanoparticleshas grown rapidly. It is therefore crucial to acquire up-to-dateknowledge of the possible harmful health effects of these materials.Since a multitude of different types of nanosized TiO2 particlesare used in industry we explored their inflammatory potentialusing mouse and cell models. BALB/c mice were exposed by inhalationfor either 2 hours, 2 hours on four consecutive days or 2 hourson four consecutive days for four weeks to several commercialTiO2 nanoparticles, SiO2 nanoparticles and to nanosized TiO2generated in a gas-to-particle conversion process at 10 mg/m3.In addition, effects of in vitro exposure of human macrophagesand fibroblasts (MRC-9) to the different particles were assessed.SiO2-coated rutile TiO2 nanoparticles (cnTiO2) was the onlysample tested that elicited clear-cut pulmonary neutrophilia.Uncoated rutile and anatase as well as nanosize SiO2 did notinduce significant inflammation. Pulmonary neutrophilia wasaccompanied by increased expression of TNF- and neutrophil attractingchemokine CXCL1 in the lung tissue. TiO2 particles accumulatedalmost exclusively in the alveolar macrophages. In vitro exposureof murine and human macrophages to cnTiO2 elicited significantinduction of TNF- and neutrophil attracting chemokines. Stimulationof human fibroblasts with cnTiO2-activated macrophage supernatantinduced high expression of neutrophil attracting chemokines,CXCL1 and CXCL8. Interestingly, the level of lung inflammationcould not be explained by the surface area of the particles,their primary or agglomerate particle size or radical formationcapacity, but is rather explained by the surface coating. Ourfindings emphasize that it is vitally important to take intoaccount in the risk assessment that alterations of nanoparticles,e.g. by surface coating, may drastically change their toxicologicalpotential.