The present invention relates to antibodies including human antibodies and antigen-binding portions thereof that bind to the extracellular domain (ECD) of activin receptor-like kinase-1 (ALK-1) and that function to abrogate the ALK-1/TGF-beta-1/Smad1 signaling pathway. The invention also relates to heavy and light chain immunoglobulins derived from human anti-ALK-1 antibodies and nucleic acid molecules encoding such immunoglobulins. The present invention also relates to methods of making human anti-ALK-1 antibodies, compositions comprising these antibodies and methods of using the antibodies and compositions. The invention also relates to transgenic animals or plants comprising nucleic acid molecules of the present invention.