Abstract: A method for selecting nucleic acid sequences encoding ligand and receptor molecules capable of specific binding to each other is disclosed in which nucleic acid encoding ligand or receptor molecules is expressed in a host microorganism in combination with a surface molecule, such as E. coli pili, so that the ligand or receptor are displayed on the surface of the host microorganism. A replicable genetic unit, such as a filamentous bacteriophage, is used to display candidate binding partners to the ligand or receptor, with the binding of the ligand or receptor to the candidate binding partner mediating the transfer of nucleic acid from the replicable genetic unit to the microorganism. The method can be highly selective as the host microorganism is modified so that it does not display the surface molecule other than as a fusion with the ligand or receptor molecule. The method is rapid and simple and opens up new applications based on the detection of ligand and receptors where both are unknown.