Information about the receptor would be useful, but is not essential.  Is the crystal structure of the ligand known? otherwise pan a peptide library with a monoclonal antibody to the ligand.

Hi Chandra, you can try to use peptide phage display from New England labs.

Dear Dr. Chandra,

This is what I understand from your question (kindly correct me if I misunderstood): You have knowledge about a ligand, which is secreted by the body and is active (agonist or antagonist) on a receptor, but the structure of the receptor is unknown, and you wish to design a mimicking peptide that would act on the receptor (either agonist or antagonist).

If what I understood is correct, here is what is needed:

I would like to know whether it is a peptide-type ligand or non-peptide-type ligand, and whether your focus is limited to only peptides or any other classes of ligand; It should be easy to develop mimicking peptides if the known ligand is also peptide-type, and it should also be easy to develop mimicking non-peptide ligands if the known ligand is a non-peptide type. Development of mimicking non-peptide ligands for a known peptide ligand is also easy, while developing peptide ligands to mimic a known non-peptide ligand can become a bit difficult task. 

You need to follow a scaffold based (also known as "ligand based") approach. You can either simply try derivatives of the known ligand or try its bioisosters or use a similar scaffold and tweak its pharmacophoric features to probe what kind of structural changes are favourable. A small library of 20-40 related compounds with sufficient diversity in pharmacophoric features should be sufficient to provide such information. From the data of bioactivity studies on this synthesized library, you would also be able to create QSAR and shape-based 3D pharmacophore model that should make the further development much easier.

Hope this helps, the attached link of a chapter on scaffold based drug discovery might be useful. 

Regards, 

Balmukund Thakkar. 

https://link.springer.com/chapter/10.1007%2F1-4020-4407-0_6