Molecular Forces.- Structure, Function and Dynamics of a Regulatory Enzyme: Aspartate Transcarbamylase.- Discussion.- Quaternary Structure of Proteins.- Subunits to Superstructures: Assembly of Glutamate De-hydrogenase.- Subunit Interactions in Haemoglobin.- Approaches for Determining Protein Complexes.- Interaction between Chymotrypsin (Trypsin) and the Pancreatic Trypsin Inhibitor (Kallikrein Inactivator).- An Example of “Quasi-Irreversible” Protein-Protein Interaction: The Trypsin-Pancreatic Trypsin Inhibitor Complex.- Subunit Interactions in Lactate Dehydrogenase.- Cooperative Association.- Analysis of the Structure of Complex Proteins by Electron Microscopy.- Probe Studies on the Role of Protein-Protein Interactions and Enzyme Conformation in Control.- Enzyme-Enzyme Interactions in Tryptophan Synthetase from E. coli.- Heterologous Protein Interactions.- Heterologous Enzyme-Enzyme Interactions.- Influences of Heterologous Protein Interactions on the Control of Glycogen Phosphorylase.- Muscle.- Protein Assemblies in Muscle.- Multienzyme Complexes.- The Pyruvate Dehydrogenase Complex of E. coli K-12, Structure and Synthesis.- The Multienzyme Systems of Fatty Acid Biosynthesis.- Antigen-Antibody Interactions.- Conformational Changes in Protein Antigens Induced by Specific Antibodies: Sperm-whale Myoglobin.- Stabilization of Conformations of E. coli ?-Galactosidase by Specific Antibodies. Restrictions in Antigenic Determinants and Antibodies.- Self-Assembly.- TMV Protein Association and its Role in the Self-Assembly of the Virus.- Intercellular Interactions.- Surface Membrane Alterations and Relevance to Cell-Cell Interaction and Growth Control in Tissue Culture.- Concluding Remarks.