A consideration of the different types of immunoelectrophoresis is the natural con- clusion to the methods of electrophoretic fractionation described in the preceding book for proteins and nucleic acids. Immunoprecipitation is the essential com- ponent process of immunoelectrophoresis, and is in itself a highly specific method of purifying proteins. It can also be used for their detection after conventional elec- trophoresis or isoelectrofocusing. It is therefore worthwhile considering the mecha- nism and specific features of immunoprecipitation before considering im- munoelectrophoresis itself. Some of these features, in particular, the ambiguity of the immune response and the polymorphism ofimmune reactions, cannot be under- stood without abrief but not superficial knowledge of the mechanism of generating immunity, the structure and function of the immunoglobulins, and the nature of antigen-antibody interactions. A knowledge of these topics is necessary for an understanding of the radioimmune methods dealt with in the second Part of the book. In the majority of cases biochemists and molecular biologists are inadequately acquainted with contemporary immunochemistry, which is currently undergoing rapid development. All this justifies the inclusion of a detailed introduction to immunochemistry as the first chapter. Chapter 2 is devoted to adescription of the use of antisera in the immunological assay of components separated by conventional electrophoresis and IEF. The remaining three Chapters analyze the different forms of immuno- electrophoresis, usually carried out in agarose gels.