PRODUCTION OF ANTIPNEUMOCOCCIC SERUM 533 



mersed in a column of bacterial suspension. A formula was derived 

 by which the reading obtained can be translated into terms rep- 

 resenting the concentration of bacteria per cubic centimeter of 

 fluid. Once this figure is determined for the suspension chosen as a 

 standard, it is necessary only to adjust each new preparation of 

 vaccine to a density affording corresponding readings by the in- 

 strument. 



By a method similar to the older technique of Hopkins, Krue- 

 ger 760 measured the cell concentration of bacterial suspensions and 

 reported a deviation of 2 to 5 per cent from actual counts. Feem- 

 ster, Wetterlow, and Cianciarulo 393 recently compared the values 

 obtained with suspensions of typhoid bacilli by direct counts, ni- 

 trogen determinations, and the Gates densitometer readings and 

 found the latter two methods to be highly correlated. Since the 

 nitrogen content is an index of bacterial mass, rather than of num- 

 bers, and hence presumably is related directly to antigenic value, 

 determinations of density by the Gates apparatus appear to af- 

 ford a simple and satisfactory means of measuring antigenic 

 substance and therefore of standardizing bacterial vaccines. A cor- 

 rected Gates reading of 2.0 centimeters on a 1 to 30 dilution of sus- 

 pension of pneumococci corresponds approximately to 25 billion 

 organisms per cubic centimeter, which furnishes a satisfactory 

 standard suspension from which measured doses of vaccine may be 

 made for routine injections. 



INJECTIONS 



Route of injection. As early as 1891, the Klemperer brothers 724 

 introduced the intravenous method of injecting pneumococcal 

 preparations, having found the immunological response to be su- 

 perior to that following subcutaneous injection. For stimulating 

 the production of pneumococcal antibodies, the intravenous route 

 for the injection of antigen is the method of choice. In the search 

 for therapeutic serum of greater efficacy, it may prove of value to 

 administer antigenic materials subcutaneously with the hope of 



