Hardly a decade ago, membrane receptors were an attractive but largely unproven concept. Since that time enormous progress has been made, and we are now able to consider receptors much more concretely. Their existence has been established, their binding properties have been determined, and in some cases, they have been highly purified and their physical-chemical properties studied. It is now even possible to visualize microscopically some receptors. This progress has resulted largely from the development of highly powerful methods. These methods are the subject of this volume. Although...