We have begun to examine the biological and toxic effects of ozone on Jurkat T cells incubated thereafter for 24, 48 and 72 h. Tissue culture medium was strengthened by adding 20% fetal calf serum with an albumin content of about 6 mg/ml. Ozonization was performed by exposing for 10 min a volume of cell suspension (4 x 105/ml) to an equal volume of a gas mixture composed of oxygen-ozone with precise ozone concentrations ranging from 1.5 up to 72 µg/ml (31.5-1512 µM). The proliferation index declined progressively and was ozone dose-dependent. The response of enzymatic activities varied depending upon the enzyme and ozone concentrations: glucose-6-phosphate dehydrogenase begins to increase at an ozone dose of 6 µg/ml (126 µM), reached a peak at 12 µg/ml (252 µM) and rapidly declined thereafter. On the other hand activities of superoxide dismutase, glutathione peroxidase and glutathione reductase increased progressively from the ozone concentration of 12 µg/ml. Thus, as we have observed in blood, the biological response is linked to the ozone dose that must reach a threshold to be effective.