Okadaic acid (OA), a diarrhetic shellfish toxin is a potent promoter of tumours in mouse skin and a specific inhibitor of protein phosphatases 1 and 2. Lipid peroxidation is one of the main manifestations of oxidative damage and has been found to play an important role in the toxicity and carcinogenesis of many xenobiotics. In the present study, we investigated the possible induction of lipid peroxidation by OA in Vero cell cultures, a cell line of renal origin, by assaying the malondialdehyde (MDA) release in the medium as well as that produced inside the cells as measured by HPLC and fluorometric quantification of the MDA-thiobarbituric acid (TBA)-adduct after extraction in n-butanol.The MDA production was largely increased by OA (0.75 ng/ml) up to 74% after 24 h incubation. Superoxide dismutase (SOD) + catalase, vitamin C and vitamin E added in the culture medium as oxygen radicals and free radicals scavengers were efficient in preventing this MDA production by OA, indicating that oxidative processes could be one of the main pathways whereby this marine toxin induces its cytotoxicity and possibly promotes tumours.