The present invention concerns the use, in a cosmetic composition, of an effective quantity of active principle originating from flax (genus Linum) to activate cytochrome c and to stimulate the functions of the mitochondria. The active principle originates from the hydrolysis of flax proteins and contains principally polypeptides or peptides. It can be used alone or in association with at least one other active principle. The invention further relates to a method of cosmetic treatment intended to protect the skin and the appendages from external aggressions and to combat against cutaneous ageing.