Ozone is known to cause radicals to be formed in biological systems: for example, it initiates lipid peroxidation and vitamin E protects in vitro model systems, cells, and animals against the effects of ozone. Ozone is not itself a radical, and we have asked: With what molecules does ozone react in the lung and how are radicals produced? Ozone reacts by two quite different mechanisms to produce radicals; one involves an ozone-olefin reaction and the other a reaction with electron donors such as glutathione (GSH). The first mechanism splits an R radical out of an olefin with the structure R-CH = CH2. The R then reacts with dioxygen to become a peroxyl radical (ROO), and both carbon- and oxygen-centered radicals can be detected by the electron spin resonance spin trap method. From the effects of temperature, metal chelators, and water, it is concluded that ozone reacts by the Criegee ozonation pathway to give the classical 1,2,3-trioxolane, which then undergoes O--O bond homolysis to form a diradical. This diradical then either undergoes beta-scission to split out the R radical or forms the more usual carbonyl oxide and a carbonyl compound. (See Figure 3 in the text). The low yield of Criegee ozonide that is generally obtained probably is due in part to the reactions forming radicals from the 1,2,3-trioxolane that compete with production of the Criegee ozonide. The second mechanism for radical production involves the reaction of ozone with electron donors. If the electron donor is, for example, GSH or its ion (GS-), this reaction produces the thiyl radical GS. and 0.3-. The ozone radical anion then reacts with a proton to form the hydroxyl radical and dioxygen: O3.- + H+-->HO. and O2. Using 5,5-dimethyl-1-pyrroline-N-oxide, the spin adduct of the hydroxyl radical is detected. Similar reactions are observed with catechol.