Abstract: The invention relates to the fields of materials sciences and medicine and relates to an agent, which can be used, for example, as a contrast medium for the localization of cancer cells. The object of the present invention is to disclose an agent which sensitively and selectively recognizes the site and the type of the molecules or cells to be examined. The object is attained through an agent composed at least of bio-shuttle molecules to which endohedral fullerenes are coupled by way of peptide-based molecules, wherein the endohedral fullerenes are hydrophobic and correspond to the formula A3-xMxZ@C2n in which x=0 to 3 and n?34, A means rare earths and/or transuranic elements, M means metals, Z means non-metals and C means carbon. The object is further attained through a method in which hydrophobic endohedral fullerenes are coupled with bio-shuttle molecules by way of an irreversible Diels-Alder reaction with an inverse electron demand (DARinv).