The disclosure relates to Lefty derivatives and the uses of Lefty polypeptides as antagonists of the function of certain ligands such as Nodal, GDF-8 (Myostatin), and GDF-11. These derivatives may be fused to other functional heterologous proteins such as IgG, especially the Fc portion of IgG. According to the disclosure, Lefty polypeptides are useful in the treatment of a variety of disorders, including, for example, neuronal diseases, muscle and bone conditions, and metabolic disorders.