After oxygen, silicon is the most abundant element in the earth's crust. Six or seven grams of silicon are present in the adult human body. The major sources are the water supply, plants and other organisms consumed as food, and chemicals or industrial processes. Remarkably little, however, is known about its biochemical roles in higher animals. This Ciba Foundation Symposium was held to examine those roles. The contributors describe how silica gets into and out of organisms of different kinds and discuss how essential or how harmful silicon or silicon-based compounds (such as aluminosilicates) are in higher animals, including humans. Kidney disease, silicosis, oesophageal cancer and Alzheimer's disease are the main conditions discussed here in which silicon in some form has or may have a harmful effect. In contrast, it is an essential trace element in the matabolism of higher animals; it may have a structural role in connective tissue; and - in the form of bioactive silicates - it is now being used for many different implants, from hip replacements and heart valves to sutures and skin grafts.