In certain aspects, the present disclosure relates to the insight that a polypeptide comprising a ligand-binding portion of the extracellular domain of activin-like kinase I (ALK1) polypeptide may be used to inhibit angiogenesis in vivo, particularly in mammals suffering angiogenesis-related disorders. The disclosure also identifies ligands for ALK1 and demonstrates that such ligands have pro-angiogenic activity, and antibodies that inhibit receptor-ligand interaction.