JOHN H. NORTHROP AND PAUL H. DE KRUIF 



659 



The stabilizing effect of the serum at pH 3 is likewise due to some 

 constitutent of the serum other than the immune body as is shown by 

 the following experiment. A suspension of Bacillus typhosus was 

 treated with an excess of immune serum in 0.10 N salt and then 

 washed once with distilled water. The suspension was then added to 



TABLE I. 



Agglutination of B. typhosus by Antityphoid Horse Serum at Various Ch. 

 G. P. A. Biiffer. 



Normal Horse Serum. 



Normal horse se- 

 rum 1.0 cc. + ty- 

 phoid suspen- 

 sions 1.0 cc 



++ 

 c. 



++ 



pH 3 buffer. The result was instantaneous and intense agglutination, 

 whereas Fig. 3 shows that in the presence of excess serum no agglutina- 

 tion occurs at a pH of 3. 



Tables I to IV show the effect of the pH on the amount of normal 

 serum and immune serum to cause agglutination of various organisms. 



