Since the discovery more than thirty years ago that antibody actlvlty could be localized to discrete plasma protein fractions, the study of immunoglobulin struc ture and function has dominated the field of immunochemistry. During this time, sources of homogeneous immunoglobulin molecules have been discovered, the subunit nature of the proteins has been defined, and the three-dimensional struc tures of the antigen-recognition portion of several antibody molecules have been elucidated. Insights into the complicated genetic control of these proteins are being gained rapidly through analysis of...