Epidemiological data have revealed significant excesses of lung cancer among workers exposed to some types of man-made mineral fibers (MMMF), but inhalation experiments performed on rats have failed to reveal the origin of these excesses. Some of these materials, however, are able, in aqueous media, to exhibit surface oxidizing properties after reduction of oxygen by a radical pathway. Tests have been carried out with 12 MMMF samples collected from various sources without prior knowledge of their origin, to determine their oxidizing surface activity in relation to their divalent iron content. Some of these fibers were obtained from factories included in an epidemiological investigation. Only fibers coming from plants where excesses of lung cancer were observed are classified as active in an oxidative process and, consequently, probably toxic in biological media by an oxidative stress mechanism. We therefore propose an hypothesis of a causal relationship between these oxidizing properties and the ability of the materials to induce lung cancer.