a small number of techniques are particularly useful in studying different types of receptors. Thus, it makes sense to speak about membrane receptor methodology. A very apparent problem in the study of membrane receptors is their presence in exceedingly small quantities and in a highly impure state. Therefore, very sensitive and specific techniques are required for their detection, characterization and purification. Such sensitivity and specificity is provided by the ability of receptors to bind certain ligands with very high affmity, and it is not surprising that most of the methods described in this volume depend upon this high affinity binding. The antigen-antibody interaction is of comparable sensitivity and speci­ ficity. Recently, a number of anti-receptor antibodies have been produced or found to occur spontaneously in auto-immune diseases. Undoubtedly, more will be produced in the future.