INTRODUCTION 25 



lour hours alter the intradermal injections, the rabbits received a single in- 

 travenous injection of 0.3 c.c. of the filtrate, per kilo of body weight. Four 

 rabbits died shortly afterwards. Nine of the surviving rabbits showed uni- 

 formly severe hemorrhagic necrosis in each of the 6 prepared areas. One 

 rabbit showed no reactions. 



Analogous results were obtained Avith other batches of identi- 

 cally prepared filtrates. 



As is seen from the abo\e experiments, it is possible to deter- 

 mine the potency of a given preparation by a reciprocal titration. 

 In order to avoid confluent reactions enough distance (approxi- 

 mately 1 to 11/2 inches) should be allowed between the sites pre- 

 pared. It is inadvisable to give more than six injections into the 

 skin of the abdominal wall, and the amount of material injected 

 should be limited to 0.25 c.c, or preferably 0.2 c.c. 



From the analysis of the reciprocal titrations it becomes at 

 once evident that there exists a distinct reciprocal relationship 

 between the amoinits of filtrate necessary to be injected intra- 

 dermally and intravenously in order to elicit the phenomenon. 

 Thus, even the highest dilution employed is able to prepare the 

 skin of some rabbits for severe hemorrhagic necrosis pr(j\'ided 

 large amoiuits of the filtrate are injected intravenotisly. As the 

 intravenotis dose is decreased, ^vell-pronoimced reactions are 

 elicited only in areas prepared ^vith louver diliuions. Moreover, it 

 is of interest to point oiU that insufficient intra\'enous doses only 

 very rarely produce ^veak reactions in all the sites prepared. In 

 most instances, the reactions appear of greatest intensity in sites 

 receiving sufficiently large preparatory doses and are totally ab- 

 sent in sites prepared ^vith lower doses. 



If the preparations employed contain a high phenomenon-pro- 

 ducing potency, the diltuion necessary for elicitation of uniform 

 reactions in all sites prepared may be beyond its lethal effect. In 

 weak preparations the lethal titer may lie very closely to the 

 phenomenon-producing titer and thus, if the necessary concen- 

 tration of the filtrate is used, a high percentage of rabbits may die 

 before the reading of the reaction becomes possible. Pabst and 

 Branham (1933) apparently attempted the above reciprocal titra- 

 tions in rabbits prepared with several injections of the menin- 

 gococcus toxic material into the skin of the abdominal wall as ^vell 

 as into the skin of both ears of the rabbit. As discussed on p. 19 

 the skin of the ears is considerably more resistant to the phe- 

 nomenon than the abdominal skin. If the intravenotis injection 



