An investigation was carried out, through laboratory experiments, into the corrosion behaviour of stainless steel pipe immersed in natural and sterile sea-water to determine whether the observed ennoble ment of the free corrosion potentials in sea-immersed stainless steels can be explained as due to the settlement of living microbiological slime on the metal surface. Experiments corroborated the fact that in natural environments, metal surface colonization by bacterial and algal populations alters the cathodic oxygen reduction process, presumably as a result of enzymic catalysis. This phenomenon not only facilitates the initiation of localized attack, but also heightens their progress and extension, thus justifying the greater aggressivity of natural, as compared with sterile, environments with equal salt content.