VII. POTENCY TESTS 655 



5x10-2 

 2 x 10-2 

 2 x 10-s 

 2 x 10-4 

 4 x 10-5 

 4 x 10-6 

 4x10-7 

 4 x 10-8 

 4 x 10.8 

 1.33x10-9 

 0.4x10-9 



This is the test dilution of culture. 



b. Serums. All dilutions of serums are made in physiological salt so- 

 lution, the amounts of diluent being drawn from an accurately gradu- 

 ated burette. Accurately graduated pipettes are used for mixing dilu- 

 tions, using a separate pipette for each mixture. Each dilution is drawn 

 up into the pipette 25 times to ensure thorough mixing. Excess serum 

 adhering to the outside of the pipette after removal from the original 

 sample is removed with a pledget of cotton. Not less than 1.0 cc. of a 

 given serum dilution is used for making any higher dilution, and the 

 volume of each dilution is kept as small as possible (usually 15 cc. or 

 less) so that mixing may be thorough. (In making the first dilution of 

 the control serum only 0.5 cc. of serum is used in order to conserve it.) 

 The test dose of all serum dilutions is contained in a volume of 0.5 cc. 

 (1) Control Serum: The control serum is one supplied by the 

 National Institute of Health and is, at present, P 11 to which have 

 been assigned unit values of 300 and 150 units per cubic centimeter 

 for Types I and II respectively. The dilutions used are for Type I, 

 1:1,000, 1:2,000, and 1:4,000; for Type II, 1:1,250, 1:2,500, and 

 1 : 5,000. These dilutions are made in the following manner : 



TYPE I 



0.5 cc. serum + 4.5 cc. saline = 1:10 (a) 

 1.0 cc. of (a) + 9 cc. saline = 1:100 (b) 

 2.0 cc. of (b) + 8 cc. saline = 1:500 (c) 

 8.0 cc. of (c) + 8 cc. saline = 1:1000 (d) 

 4.0 cc. of (d) + 4 cc. saline = 1:2000 (e) 

 2.0 cc. of (d) + 6 cc. saline = 1:4000 (f) 



