It is well established that many different types of fibre, both organic and inorganic, can become coated with ferritin in lung to give ferruginous bodies. The presence of such bodies is one factor which is taken into account in assessing cases with asbestos-related disease for the purpose of compensation, although their presence is only an indication of past exposure and not proof of the causative role of asbestos. In this paper, evidence relating to the formation of ferruginous bodies in human lung is reviewed, to identify factors which predispose certain types and dimensions of fibre to become coated. Work with experimental animals is also summarised to give some indication of the time course of body formation.