Electron Spin Resonance (ESR) spectrometry has been widely used to assess and quantify free radical production in cigarette smoke and its separated gazeaus phase (reviewed in CULCASI M. & PIETRI S. - 1999 - Bull. ARN, 24-47). In cultures of murine fibroblasts (3T3 cells) exposed to cigarette smoke or its gaz component (from CM2 reference cigarettes), several free radical species such as O₂· ^- (superoxide), HO· (hydroxyl), CO₂ · ^- , R· (alkyl) can be detected by using ESR spin trapping with the nitrone spin trap 5-diethoxyphosphoryl-5-methyl-pyrroline-1-oxyde (DEPMPO). In addition to these latter species that are formed in cigarette smoke, the glutathione-derived free radical GS· can also be observed in cell cultures as a specific index of smoke-induced cellular injury. Cytotoxicity measurements (lactate dehydrogenase assay, Lowry test) during exposure of the cells to cigarette smoke or its filtered gaz phase revealed marked differences in the kinetics of oxygen-derived free radical formation and cell survival. The use of ESR spin trapping in cellular systems therefore appears as a promising tool to investigate the relationship between cigarette smoke-induced free radical formation and toxicity.