It is hardly necessary to define the concept of receptors to readers of this series, but it should be borne in mind that in several instances receptors are undefined entities, whose molecular details remain to be established. On the other hand the ligand, which recognizes 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 we may understand in the near future the details of ligand-receptor interaction. This interaction involves two defined steps: the first, usually referred...