Information concerning the carcinogenicity of bracken fern (Pteridium aquilinum) in different species and organs is reviewed. Various methods used in attempts to identify the plant components responsible are described and the results evaluated. Reference is made to recent and current research in this field (University College of North Wales, Bangor) and includes mutagenic and sterility studies, and the introduction of teratogenic screening by injection of Japanese quail eggs. The many problems still challenging bracken workers are indicated, and a final section deals with the potential hazard to human health in an attempt to link experimental work with epidemiological surveys of human cancer incidence. One important result of the study of bracken carcinogenicity is that active components, previously thought to be harmless, can be found in a wide variety of regular dietary items.