Abstract: The invention relates to a method for assaying for the presence of a substance that modulates a cytokine of the TGF.beta. superfamily. A substance which is suspected of modulating a cytokine of the TGF.beta. superfamily and a TGF.beta. binding compound which is not a TGF.beta. receptor and which contains a TRH1 domain, or a portion or mimetic thereof, is reacted with a cytokine of the TGF.beta. superfamily under conditions where the compound, portion or mimetic thereof, and the cytokine are capable of forming a complex. Complexes, free compound and/or cytokine are assayed and compared with a control. The invention also relates to a composition comprising at least one compound which is not a TGF.beta. receptor and which contains the TRH1 domain or a portion, or a mimetic thereof, and a pharmaceutically acceptable carrier, auxiliary or excipient and to methods of treatment using the composition. Further the invention relates to a method of enhancing the activity of growth factors.