Anatase and rutile are titanium dioxides (TiO2) with different crystal lattices. The particles of TiO2 are considered a "nuisance" dust. It has been reported that rutile can be considered "inert". However, anatase, because of its hemolytic activity in vitro and slow lung clearance, should warrant further research regarding its toxicity. We exposed rats to an aerosol of either anatase or rutile and determined the TiO2 retention in the lung up to 132 days post exposure. Particle clearance from the lung, calculated from the retention data, was similar in both the anatase and the rutile groups with T1/2 of 51 or 53 days, respectively. In addition a pulmonary cell response test was performed on other rats. After intratracheal instillation of anatase and rutile in doses of 0.5 or 5.0 mg/rat, lung lavage was performed and the harvested cells counted. The counts of all cells, alveolar macrophages (AM), peroxidase positive AM, and polymorphonuclear leukocytes were compared. The pulmonary cell response test also yielded similar results for both types of TiO2. Thus there was no indication that the crystal lattices of TiO2 altered the biological effects of TiO2 particles. The evidence suggests that both anatase and rutile are "nuisance" dusts.