The study presented in this thesis was performed in order to identify ligands for VEGFR-3 and to characterize their structure and function. The hypothesis that VEGF-C is a growth factor for lymphatic endothelial cells was tested using two different in-vivo models. Subsequently, it was shown that a novel VEGFR-3 ligand - VEGF-D - has the same receptor-binding pattern as VEGF-C. Finally, the structural determinants of VEGFR-3 binding were characterized in relation to VEGF. This approach led to the identification of VEGF/VEGF-C mosaic molecules with novel receptor binding profiles, and a panel of these molecules was used to delineate the requirements of specific receptors in the induction of angiogenesis versus lymphangiogenesis in the chorioallantoic membrane.