We propose to complete the determinations of the three-dimensional structures of the serum IgGl immunoglobulin and the urinary Bence-Jones protein from a patient (now deceased) with multiple myeloma and amyloidosis. We shall continue our long-standing collaboration with H.F. Deutsch and his colleagues, who have performed parallel amino acid sequence studies and provided continuing insight into the immunological and genetic implications of our work. The high-resolution (2.3-A) structure of the Bence-Jones (light chain) dimer will be refined mathematically to improve the accuracy of the atomic model of 3200 non-hydrogen atoms. The improved model will be used to check and extend our previous correlations of structure with the binding of the hapten-like molecules in a cavity similar to those in antigen-binding (Fab) fragments. We shall continue to explore the conformational isomerism of the light chain monomers and the rotational allomerism of the amino ("variable") and carboxyl ("constant") domains of individual light chains. Such isomerism and allomerism have profound implications in the functions and evolution of immunoglobulins, and help explain the observations of the groups of Glenner, Franklin, and others on the formation of amyloid fibrils of immunoglobulin origin. The methods and results of the Bence-Jones project will be applied to the technically more imposing problem of the serum IgGl molecule, in which the light chains have the same amino acid sequence as the Bence-Jones monomers. As a prerequisite to the proposed studies, new methods are currently being developed for the preparation of crystals and isomorphous heavy atom derivatives. Crystals grown in water are markedly sensitive to fluctuations in temperature and humidity, and are subject to dissolution or disorder when salts of heavy atoms are added in concentrations as low as 0.001 M. Methods for careful data collection under controlled conditions of temperature and humidity are being tested. Solutions to these problems will require all of the expertise we have accumulated, but should be expedited by the proximity and proferred assistance of protein chemists like W.R. Gray and S. Velick of Utah.