It is hardly necessary to define the concept of receptors to readers of this series, but it should be born in mind that in several instances receptors are undefined entities, whose molecular details remain to be established. On the other hand the ligand, which reco izes the receptors, has been identified and characterized in most cases. The current interest in the structure and function of biological membranes gives great expectations that in the near future we may understand the details of ligand-receptor interaction. This interaction involves two defined steps: the first, usually referred...