The very title of this monograph brings to mind classic controversy. Is the rarity of a second attack of syphilis due to acquired immunity or to incomplete cure of the original infection? What is the nature of the so-called antibodies in syphilis? These and other questions relating to syphilis in man as well as in monkeys and rabbits are discussed in a critical and readable manner. The author is well known by his extensive experiments in immunity in syphilis carried out in collaboration with associates at the Johns Hopkins Medical School, and he is especially fitted to discuss this subject. The volume combines a brief summary of the old views and an extended summary of modern opinion, based on recent investigations, and should be of great interest to students of syphilology.