Individualized, chemically pristine single-walled carbon nanotubes have been intravenously administered to rabbits and monitored through their characteristic near-infrared fluorescence. Spectra indicated that blood proteins displaced the nanotube coating of synthetic surfactant molecules within seconds. The nanotube concentration in the blood serum decreased exponentially with a half-life of 1.0 ؎ 0.1 h. No adverse effects from low-level nanotube exposure could be detected from behavior or pathological examination. At 24 h after i.v. administration, significant concentrations of nanotubes were found only in the liver. These results demonstrate that debundled single-walled carbon nanotubes are highcontrast near-infrared fluorophores that can be sensitively and selectively tracked in mammalian tissues using optical methods. In addition, the absence of acute toxicity and promising circulation persistence suggest the potential of carbon nanotubes in future pharmaceutical applications.nanoparticle biodistribution ͉ nanoparticle toxicity ͉ luminescence spectroscopy ͉ single-walled carbon nanotubes S ingle-walled carbon nanotubes (SWNTs) are an important class of artificial nanomaterials with remarkable mechanical, thermal, electronic, and optical properties. These properties suggest diverse future biomedical uses in areas such as targeted chemotherapeutics, in vitro cell markers, diagnostic imaging contrast agents, biochemical sensors, and photoablative therapy agents (1-9). Before medical applications can be developed, it is necessary to explore the behavior and fate of SWNTs in mammals. However, little is currently known in this area, in part because of the challenge of detecting and tracking these allcarbon nanoparticles in complex biological environments.SWNTs can be envisioned as sections of graphene sheets rolled up to form seamless cylindrical tubes with a variety of structures (10). Each of these structures has a well defined diameter and chiral angle and shows either semiconducting or metallic behavior. The nanotube preparations used for this study contain several dozen structural types that are Ϸ1 nm in diameter and Ϸ300 nm long. After excitation with visible light, each type of semiconducting SWNT fluoresces at a near-infrared (near-IR) wavelength between Ϸ900 and 1,600 nm that is characteristic of its specific structure (11, 12). We have previously exploited this fluorescence emission to study the active ingestion of SWNTs by macrophage cells in vitro (13).Here we report the use of the intrinsic near-IR fluorescence, which is a property only of individualized SWNTs, to measure their blood elimination kinetics in rabbits and to identify the organs in which they concentrate. These methods and results provide a foundation for developing the targeted delivery of nanotubes to specific tissues for diagnostic and therapeutic uses. In contrast to alternative methods that track carbon nanotubes by linking them covalently or noncovalently to external fluorophores or chelated radioisotopes (1,8,14), the near-IR fluor...