Dr. Kahn's interest in the universal serologic reaction really began about thirty years ago, when he observed, in studying sera from syphilitic patients, varying patterns of precipitation obtained (1) by using different concentrations of sodium chloride and (2) by incubating tests at temperatures colder than 37 C. Subsequently, certain other patterns of precipitation were noted in nonsyphilitic (false-positive) sera. Through the years this investigator has studied hundreds of syphilitic and nonsyphilitic sera from man and animals. This volume represents a comprehensive report of universal serologic reactions observed in (1) normal human beings, (2) animals, such as rabbits, mice, guinea pigs, cows, and dogs, (3) syphilitic human beings, and (4) patients with yaws, leprosy, malaria, tuberculosis, and defective development. Dr. Kahn's thesis is that certain lipid substances are normally bound in tissue and are liberated during ordinary wear and tear; these substances act as antigens, probably stimulating production of reactive antibodies,