Metallic gold (Au੠is a likely biotransformation product of monovalent gold, Au(I) whenever it is dissociated from in vivo ligands, Auࠢeing formed either by bioreduction or by spontaneous dismutation (with co-production of trivalent gold). This review discusses the preparation and some biologically relevant properties of colloidal metallic gold (CMG) in its nano-particulate form. Tyndall's purple, a well characterised preparation of CMG, shows potent anti-arthritic activity in rats, approximately 103 times that of sodium aurothiomalate (Myocrysin). Even more remarkable is its broader spectrum of action in rats compared to this classic DMARD.