Pteridium aquilinum also called bracken fern is one of the most common weed plants in the world. Its propagation is only limited by extreme kinds of temperature or humidity. Due to changing agricultural factors; farm animals are sometimes forced to consume huge amounts of bracken fern. Humans also are in contact with it either directly, by eating the fronds and rhizomes or indirectly by consuming bracken fern infested milk. As a result, farm animals show a series of known syndromes due to the toxic substances of bracken fern. These include; thamine deficiency of monogastric animals, acute haemorrhagic syndrome together with bone marrow aplasia and upper alimentary ulceration, retinal atrophy in sheep and two neoplastic disease syndromes. These two syndromes consist of neoplasia of the urinary bladder leading to haematuria and a syndrome involving upper alimentary carcinomata. The main reason for these neoplastic formations is the nor- sesquiterpenoid type of a glycosid called ptaquiloside. Also other carcinogens and mutagens seem to play a role especially in association with papilloma viruses. Epidemiological analyses showed that the risk of cancer in humans is increased by direct or indirect consumption of bracken fern. Ptaquilosides are forming a DNA-Adduct which interacts with genetic material, especially adenin which then subsequently causes abnormal mutations. Experimental trials on mice determined the first steps in the control of the immunotoxic effect induced by ptaquilosides and proved that selenium has a preventive role as well as a reversible effect.