TREASURY DEPARTMENT. 
Public Health and Marine-Hospital Service of the United States. 
Walter Wyman, Surgeon-General. 
HYGIENIC LABORATORY.— BULLETIN No. 21. 
1905 . 
THE IMMUNITY UNIT 
FOR STANDARDIZING 
DIPHTHERIA ANTITOXIN 
(BASED ON EHRLICH’S NORMAL SERUM) 
Official Standard prepared under the Act approved July 1, 1902.. 
BY 
iVl. J. ROSEX^U, 
DIRECTOR OF THE HYGIENIC LABORATORY. 
WASHINGTON: 
GOVERNMENT PRINTING OFFICE. 
