SERUM DIAGNOSIS 159 



der to determine the unit. A unit of hemolytic 

 amboceptor is the smallest am.ount that gives 

 complete hemolysis of 0.1 c.c. of a 5 per cent 

 suspension of sheep's red cells, in the presence 

 of an excess of complement, after one hour's 

 incubation in a 37 C. water bath. A unit of 

 antigen is the smallest amount which, with two 

 units of hemolytic amboceptor, gives complete 

 fixation of complement. 



MAKING THE TEST 



In carrying out the test, the reagents are 

 mixed in small glass tubes of about 2 c.c. capac- 

 ity which are placed in suitable racks. Various 

 dilutions of the immune sera are first made. 

 Definite quantities accurately measured by 

 special pipets of serum, antigen, and com- 

 plement, are placed in the series of glass tubes. 

 The mixtures are thoroughly shaken and the 

 racks containing the tubes are placed in a water 

 bath for one-half hour at 37 C. No test is of 

 value unless suitable controls are made. For 

 this purpose, tubes containing known positive 

 serum, tubes containing normal serum, and 

 tubes without any serum but containing an ex- 

 cess of antigen, are run as controls. If the 



