Intraperitoneal inoculation of the mouse has been employed for many years as a means of assaying the virulence of different strains of Salmonella typhi or of detecting alterations in virulence in the same strain occurring from time to time in the laboratory. Such changes are usually in the nature of loss of virulence resulting from antigenic change. Occasionally, however, there may be increase of virulence, either as a result of mouse passage or simply from a more strict observance of nutritional requirements and improvements in the method of cultivation. Where cultures of typhoid organisms are to be used for vaccine production the detection of these antigenic changes and the associated loss of virulence is obviously of the greatest practical importance.