Titanium dioxide (TiO2) dusts exhibit a hemolytic behavior against human erythrocytes in vitro. The hemolytic activity depends on the crystal lattice of TiO2—rutile being practically inert and anatase having significant activity. In a preliminary in vivo experiment both types of TiO2 failed to increase the proline hydroxylase level in rat lung when measured four weeks after TiO2 inhalation, although proline hydroxylase levels have been shown to increase in the initial stages of fibrosis. This negative finding does not, however, eliminate the possibility of fibrogenicity because the incubation period was short. The retention of anatase dust in the lungs was also remarkably greater than that of rutile. Because of the found properties of anatase, more extensive studies should be performed to determine the possible biological effects of inhaled TiO2, especially those of anatase.