Professor Kuczynski submits a very elaborate summary of extensive studies in experimental yellow fever. Though elaborate, it is still a summary in that the details of all experiments are not given. Thousands of original cultures were made and hundreds of experimental animals, monkeys, were used. By means of graphs and tables the pertinent results are presented in a very concise and understandable manner. The most interesting feature of the book and the chief purpose of the author is the marshalling of the evidence he has collected to indicate that the cause of the disease is an organism isolated many times during the work with yellow fever in monkeys. Because of the characteristic damage to the liver in yellow fever, a condition which has been termed hepatodystrophy, he has named the organism B. hepatodystrophicans. It appears to belong to the group Corynebacterium, i.e., a diphtheroid.