AbstractBracken fem (Pteridiumspp.), one of the most abundant plants on the planet, is well known to cause cancer naturally in sheep and cattle. It contains, in some locations, extremely high concentrations of ptaquiloside which almost certainly is its major carcinogen. Ptaquiloside is transferred through milk. There is epidemiological evidence that the bracken carcinogen, in special situations, may cause cancer in man. Ptaquiloside and its animal models of carcinogenesis also offer good tools for the study of cancer. © 1995 Wiley‐Liss, I