- We have expressed the extracellular binding domain of the human LH/CG receptor as a fusion with the cpIII filamentous phage coat protein. These fusion phage are able to specifically bind ELISA plates coated with hCG. Preincubation of the phage with hCG before exposure to the coated plates inhibited binding of phage, whereas human FSH had no effect. Furthermore, addition of anti-hLH/CG receptor antisera inhibited binding of phage to the plates. We have also tested the effect of monoclonal antibodies to hCG on phage binding. Monoclonal antibodies that can bind hCG when it is bound to native receptor do not inhibit phage binding. These include B105 and A109 Alternatively, B107 and B109 have an inhibitory effect on phage binding. They cannot bind hCG when it is bound to receptor and are likely directed against epitopes at or near the receptor binding interface. Therefore, these fusion phage exhibit the same binding specificity as the wild-type receptor and likely display the extracellular domain of the hLH/CG receptor on their surface in the native conformation. Phage display is a potentially useful tool for studying the hLH/CG receptor structure and in screening for synthetic compounds that compete with hormone for receptor binding.