We have attempted to evaluate whether, similar to the angiogenesis of blood vessels, cultures of lymphatic endothelial cells (LEC) isolated from dog thoracic ducts have an ability to induce lymphangiogenesis in response to basic fibroblast growth factor (bFGF), then to examine the effects of heparin on the bFGF-me-diated morphogenesis. The effects of bFGF and/or heparin on the proliferation and migration of the LEC were evaluated by changing the num-ber of the subconfluent cells and by wound mi-gration assay, respectively. The effects of the agents on invasion and tube formation of the LEC into a three-dimensional collagen gel and on collagen gel induced tube formation of the LEC were also investigated by a phasecontrast microscope and an electron microscope. The bFGF (10nq/ml) caused a siqnificant induction of proliferation and migration of the LEC, the induction of which was augmented dose-dependently by an additional treatment with heparin ranging from 1 to 100ug/ml. The bFGF produced invasion and tube formation of the LEC into a three-dimensional collagen gel. The bFGF also facili-tated to form capillary-like tubes of the LEC between two layers of collagen gels. Heparin (10ug/ml) accelerated both processes of bFGF-mediated lymphangiogenesis of the LEC. These findings suggest that the cultured LEC isolated from dog thoracic ducts have an ability to form lymphatic capillary-like tubes in response to bFGF and that heparin accelerates dose-depen-dently the process of the bFGF-mediated neovascularization of lymph vessels. [Japanese Journal of Physiology, 48, 133-141, 1998]