THE conjecture that atoms can be trapped inside closed carbon cages such as the fullerenes was first made by Kroto et al1. Mass spectroscopic evidence obtained soon after2 suggested that lanthanum atoms were encapsulated in fullerenes prepared by laser vaporization of a lanthanum-impregnated graphite disk, and these results were later corroborated3,4. Recently, helium atoms have been incorporated into fullerenes through collisions in the gas phase5, and evidence has been obtained for the formation of metal-containing fullerenes during arc burning of composite graphite rods6. All of these studies, however, have produced quantities too small for characterization using standard spectroscopic techniques. We report here the preparation of milligram quantities of lanthanum-containing C82, which can be solvent-extracted in yields of about 2% along with empty C60 and C70 cages. We have measured the electron paramagnetic resonance spectrum of this mixture, both in solution and in the solid state, which reveals that the lanthanum atom has a formal charge of 3+, and the C82a charge of 3-. This runs contrary to some expectations that the doubly charged f ulleride anions would be the most stable species6,7; it also reveals that the fullerene cages have the same formal charge as in the superconducting alkali-metal-doped phases8,9. © 1992 Nature Publishing Group.