The immunology of Hemophilus pertussis has been reinvestigated during the last few years by several experimenters who have essentially verified and extended the original observations of Bordet and Gengou. Prophylaxis with whole-cell vaccine such as that specially prepared according to Sauer (1), and therapy with Pertussis Undenatured Bacterial Antigen as prepared by Krueger's technic (2, 3) have coincidentally come into considerable clinical use. While whole-cell vaccine can be standardized by bacterial count or turbidity, such methods are obviously not applicable to the clear ultrafiltered preparations of Undenatured Bacterial Antigen, and total-nitrogen determinations have been the principal means of standardizing the latter antigen (3). We have tested occasional lots of the antigen by active immunization of mice (4–6), but these methods are quite time-consuming and the antigen is subjected to added ageing while being tested. Further experiments, therefore, have been directed toward the determination of precipitable nitrogen as well as total nitrogen, and measurement of antigenicity by means of serological tests.