OBJECTIVE The objective of this work was to explore a new modality of cell tracking that uses multifunctional nanoparticles. The tracking of transfused cells in vivo is an important step to study the therapeutic course and mechanism of cell therapeutics. BACKGROUND Several methods of cell tracking have been developed. Our novel method uses multifunctional nanoparticles to track cells via both fluorescence and magnetic resonance. METHODS Poly (N-isopropylacrylamide) (PNIPAM)-coated multifunctional nanoparticles containing both CdTe quantum dots (QDs) and Fe(3)O(4) magnetic particles were used to label Chinese hamster ovary (CHO) cells. The labeled cells were measured by confocal fluorescence microscope in vitro at the photoluminescent band (600 nm) of QDs. When these labeled cells were injected into the mouse, the in vivo images were detected by magnetic resonance imaging (MRI). RESULTS The nanoparticles easily bound to the plasma membrane of CHO cells after incubation at 37 degrees C. The surface PNIPAM of nanoparticles is a well-known thermoresponsive polymer with a volume phase transition temperature. It is hydrophilic at temperatures below critical solution temperatures (32-34 degrees C) and becomes hydrophobic at higher temperatures. The cellular binding of nanoparticles was stable and nontoxic, and the photoluminescence of nanoparticles could still be seen after 48 h in labeled cells. In addition, the labeled cells can be manipulated by an external magnet. The magnetic resonance images showed that these labeled cells also can be measured in vivo in mice. CONCLUSIONS PNIPAM-coated multifunctional nanoparticles showed potential for labeling cells and for tracking cells both in vitro and in vivo with the use of fluorescence and magnetic resonance. This new modality of cell tracking has the merits of simplicity and reliability.