Toxicity studies of gold nanoparticles in mouse organs. Histological specimens of mice tissues (lung, kidney, liver and spleen) collected from mice euthanized on day 15, stained with hematoxylin and eosin (H and E) showed normal histology. 100% long-term survival of mice was also observed in the mice treated with gold nanoparticles at a concn of 500 nm for 15 days. A. Control animal lung section showing normal alveolar geometry and normal appearing alveolar septum. B. gold treated animal lung section showing normal alveolar membranes with normal parenchyma blood vessels. C. Control kidney section showing normal renal cortex and glomerular tufts D. gold treated kidney section showing normal glomerular tubules and renal cortex E. Control animal liver section showing normal, central vein and hepatocytic architecture F. Gold nanoparticles treated liver also showed normal hepatocytes with clear central vein G. Spleen sections of control animals showing normal splenic architecture with normal lymphoid follicles and sinuses H. gold treated spleen showing no pathological changes.