Members of the TGF-.beta. superfamily control many physiologic and pathophysiologic processes in multiple tissues and signal via type II and type I receptor serine kinases. Type II activin receptors are promiscuous and known to bind 12 TGF-.beta. ligands including activins, myostatin, BMPs and nodal. Methods are described for the screening and identification of antagonist for TGF-.beta. superfamily members, in particular activin-A antagonist.