BRACKEN (Pteridium aquilinum) and extracts of the plant have pluripotent carcinogenic and mutagenic properties, and if administered in sufficient concentration are acutely toxic to species as varied as mice and bovines1–9. The causative factor(s) of these different biological activities seem to be identical; they are produced by the same purified fraction although obtained by different fractionation procedures5,10–12. This is not surprising because the acute lethal syndrome is radiomimetic in nature, being characterized by bone marrow aplasia, haemorrhage and severe gastro-intestinal damage, together with other systemic changes common to both syndromes13–15.