This is the printed record of a symposium held under the auspices of the New York Academy of Medicine. The contributors are outstanding workers in their field. The scope of the work is best indicated by a list of some of the contributors and the subjects about which they wrote. Felix Haurowitz, Indiana University, presented "Theories of Antibody Formation"; Jules Freund, Public Health Research Institute of the City of New York, "The Response of Immunized Animals to Specific and Nonspecific Stimuli"; Michael Heidelberger, College of Physicians and Surgeons, Columbia University, "Persistence of Antibodies in Man After Immunization"; Elvin A. Kabat, College of Physicians and Surgeons, Columbia University, "The Unity and Diversity of Antibodies"; A. M. Pappenheimer Jr., New York University College of Medicine, "Valence of Antibodies"; Francis C. Lowell, Robert Dawson Evans Memorial Hospital and Boston University School of Medicine, "Skin-Sensitizing Antibody and Allergy and Resistance to Insulin"; Merrill W.