Modification of proteins by reactive oxygen species is implicated in different disorders. The proteasome is a multicatalytic proteinase in charge of intracellular protein turnover and of oxidized proteins degradation. Consequently, proteasome function is very important in controlling the level of altered proteins in eukaryotic cells. Evidence for a decline in proteasome activity during skin photo-aging has been provided by us in Bulteau et al. (2002). We previously described the ability of a lipid algae extract (Phaeodactylum tricornutum) to stimulate 20S proteasome peptidase activities (Nizard et al. 2002). Furthermore, keratinocytes treated with Phaeodactylum tricornutum extract and then UVA and UVB irradiated, exhibited a sustained level of proteasome activitiy comparable to the one of non-irradiated cells. The level of modified proteins can be quantified by measurement of protein carbonyl content (Oxyblot technique), which has been shown to increase with aging and other disorders. Here, we describe that, in the presence of this lipid algae extract, the level of oxidized proteins is reduced as assessed by the Oxyblot technique. These results are obtained both with culture human keratinocytes and stratum corneum skin cells (obtained by stripping) from human volunteers. Altogether, these results argue for the presence of compounds in this algae extract that have a stimulating and/or protective effect on proteasome activity resulting in a decrease level of protein oxidation.