The ozonide derived from methyl linoleate was shown to cause a dose dependent inhibition of the phagocytosis of rat alveolar macrophages exposed in vitro to concentrations varying from 10−5 to 10−4 M. Vitamin C was demonstrated to detoxify the ozonide. In analogy to their behaviour on exposure to ozone, vitamin E supplemented cells demonstrated a decreased and glutathione depleted cells an increased sensitivity towards the compound. The characteristics of antioxidant protection of cells against the ozonide were thus comparable to those for protection against ozone. Preincubation with glutathione also detoxified the ozonide model compound. Survival of rat alveolar macrophages exposed to a toxic concentration of the ozonide (86 μM final concentration), measured by phagocytosis of the cells, increased significantly (P < 0.01) from 23 to 54% after a 2.5-h preincubation of the ozonide with glutathione (5 mM final concentration). The detoxification of methyl linoleate ozonide by glutathione could be catalyzed by the rat liver glutathione S-transferases. After a 2.5-h preincubation of the ozonide (86 μM final concentration) with glutathione and glutathione S-transferases final concentrations, respectively, 5 mM and 0.01 mg/ml), its toxicity was completely abolished, as demonstrated by the 98% survival (P < 0.001) of subsequently exposed cells. A Km(app) (at 1 mM glutathione) for the ozonide of 0.80 mM and a Vmax(app) (at pH 6.5) of 94 nmol glutathione converted · min−1 · mg protein−1 or (at pH 7.4) of 34 nmol glutathione converted · min−1 · mg protein−1, were found. This glutathione S-transferase catalyzed detoxification of the potential intermediates in ozone induced cell damage, offers a new viewpoint on the role of glutathione in the protection of cells against ozone.