An exposure system has been developed in which direct interaction between ozone and cultured cells can be realized by gas diffusion through thin teflon films supporting the cells growing under normal tissue culture conditions (Alink et al., Chemosphere 2, 63-73, 1979). Confluent monolayers of human alveolar type II cells (A549) were exposed to 0.1 or 0.2 micrograms ozone/culture dish (growth area 12 cm2) in 2.5 h. Viability and plating efficiency were decreased at both concentrations, as compared to control cells. After exposure to 0.1 microgram ozone the superoxide dismutase (SOD) activity was higher than in unexposed cells due to an increase of the mitochondrial component of SOD. Morphological studies of the cells after exposure to ozone, using light microscopy, scanning and transmission electron microscopy, revealed alterations like a rounded shape, disappearance of microvilli and disintegration of the cytoplasm. In spite of these effects, the plasma membrane showed no ultrastructural damage, suggesting that the primary reaction with ozone had taken place at the molecular level. These results further indicate that exposure of cultured cells to ozone by means of gas diffusion through teflon films may contribute to the evaluation of the toxic effects of this pollutant in vitro.