We describe the in vivo fluorescence imaging method of novel threadlike tissues (Bonghan ducts) inside the lymphatic vessels of rats with fluorescent magnetic nanoparticles. This threadlike structure was not visible by a stereomicroscope because of its transparency. Its thickness was about 20 mu m and floated in the lymphatic fluid without adhering to the lymphatic wall. Injecting the nanoparticles into the lymph nodes and applying static magnetic fields on the lymphatic vessels, we were able to obtain the in vivo fluorescence imaging of the threadlike structures under the fluorescence reflectance imaging system. The specimen was analyzed with a confocal laser scanning microscopes and transmission electron microscopes which exhibited the preferential absorption of the nanoparticles by the threadlike structures compared with the lymphatic walls. This preferential absorption was due to the loose extracellular matrix of the threadlike structures. These results show new applications of nanoparticles for in vivo imaging of hardly detectable tissues using fluorescence reflectance imaging and magnetophoretic control. (c) 2006 Elsevier B.V. All rights reserved.