This paper describes the in vivo behavior and potential metabolism of C 60 and a more water-soluble quaternary ammonium salt-derivatized C 60 . In both cases, a 14 C-labeled fullerene core was utilized for the target molecules that were intravenously injected into female Sprague-Dawley rats. The 14 C-labeled C 60 (*C 60 ) was rapidly (within 1 min) cleared from the circulation and the majority of the *C 60 accumulated in the liver (90-95%). *C 60 was not eliminated from the liver over the 120-h period of this study. Our results also suggest that C 60 is not metabolized by the typical oxidative patterns characteristic of other polycyclic aromatics. Therefore, although not acutely toxic, use of C 60 , or its derivatives that could be cleaved back to the parent C 60 in vivo, would likely lead to long-term fullerene accumulation in the liver. The uptake of *C 60 and 14 C-labeled ammonium salt-derivatized C 60 (1) by human keratinocytes in vitro showed that while both *C 60 and 1 are readily taken up by cells, 1 accumulates more slowly. Additionally, while C 60 , at rather high concentrations (2.0 ศM) and over extended periods of time (8 days), is able to inhibit the growth of human keratinocytes by about 50%, this effect showed little, if any, photoinducability.