The potential protective effects of the flavanol catechin, the flavonol quercetin, the flavones, luteolin and rutin, and the isoflavones, genistein and daidzein, against the photo-oxidative stress induced by ultraviolet A radiation (UVA) and by phototoxic reactions resulting from the interaction of UVA with drugs and chemicals, has been assessed with cultured human skin fibroblasts. Lipid peroxidation and cell death have been chosen as model photobiological damage induced by UVA alone or photosensitized by cyamemazine (CMZ) and its photoproduct possessing phototoxic properties. Contrasting effects of flavonoids are observed. The flavanol, the flavonol and the flavones may protect against lipid peroxidation and cell death induced by 30 J cm(-2) of UVA alone or CMZ plus 10 J cm(-2) UVA. On the other hand, an amplification of the photodamage may be observed with isoflavones. A concentration-dependence study demonstrates that among the protective flavonoids, quercetin is the most efficient. The very effective protection brought by quercetin may result from its ability to scavenge reactive oxygen species produced by the photo-oxidative stress. However, the modification of membrane properties and the alteration of the lysosomal function by quercetin may not be neglected in these protective effects. The amplification of the photodamage by isoflavones is in sharp contrast with previous literature data demonstrating photoprotection by genistein. As a consequence, it may be concluded that an eventual antioxidant action of genistein may strongly depend on cells and photosensitizers. Furthermore such contrasting pro-versus anti-oxidant effects have to be taken into account when using flavonoid mixtures of plant extracts.