Gadonanotubes (GNTs), which are powerful new T(1)-weighted MRI contrast agents, were derivatized with serine amino acid substituents to produce water-soluble (2 mg ml(-1)) ser-gadonanotubes (ser-GNs) as magnetic nanoprobes for intracellular labeling. The ser-GNTs were used to efficiently label MCF-7 human breast cancer cells (1.5 x 10(9) Gd(3+) ions/cell) with no observable cytotoxicity. Cell pellets derived from the ser-GNT labeled cells give bright T(1)-weighted MR images, confirming that the ser-GNTs are a promising new nanoprobe technology for magnetic cell labeling and possibly for in vivo cellular trafficking.