The role of complement in bacteriolytic processes is familiar. Recent investigations emphasize the importance of the complement of the blood in the parenteral digestion of proteins and in infections. Thus Friedberger and his associates1 produced anaphylatoxic substances from bacterial and other proteins by means of the action of complement. Variations in the complement content of the blood during disease have been noted by Moro,2 who concluded that a capacity for the ready formation of complement was of prognostic value. I have found3 a decrease in hemolytic complement before the crisis and an increase at the time of crisis in pneumonia. Liidke and others4 found a decrease in complement in chronic suppurative processes. Eliasberg5 found the blood of leprous patients poor in complement. In view of the possibility of raising the complement content of the blood for therapeutic purposes, it is rather surprising that there is so little unity of opinion either as to the origin of complement or as to the nature of the chemical processes with which it is concerned.