STANDARDISATION OF ANTITOXIN 283 



amount of the toxin broth which, when mixed with one 

 unit of antitoxin, just suffices to kill a 250-grm. guinea- 

 pig on the fourth or fifth day after the injection of the 

 mixture ; this amount of toxin is known as the L + dose. 

 The L + dose may be defined as that amount of a given 

 diphtheria toxin broth which is not completely neutralised 

 by one " unit " of standard antitoxin to the extent that 

 exactly one simple lethal dose of toxin remains unneutralised ; 

 it corresponds usually to 105-120 minimal lethal doses. 

 For example, suppose 0'003 c.c. of the toxin was found 

 to be the minimal lethal dose, with separate " units " of 

 standard antitoxin, 0*2, 0'3, 0*4, and 0'5 c.c. respectively 

 of the toxin might be mixed, and each mixture injected 

 into a guinea-pig ; probably the guinea-pigs receiving the 

 " unit " of antitoxin plus 0'2 and 0'3 c.c. of toxin would 

 remain alive, while the animal receiving the 0'4 c.c. of 

 toxin would die in twenty-four to forty-eight hours. The 

 death in the last case is too rapid ; more than a simple 

 lethal dose has remained unneutralised, and therefore the 

 L + dose of toxin lies between 0'3 and 0'4 c.c., and further 

 experiments would have to be performed with amounts 

 of toxin between these limits in order to ascertain the 

 exact dose. Death of the guinea-pig on the fourth or 

 fifth day has been chosen because it has been found that 

 if the dose of toxin be diminished ever so little below that 

 producing this result, death does not ensue under nine or 

 ten days. That is to say, an acute intoxication is fatal 

 at the latest on the fourth or fifth day, a fatal result after 

 then being due to a chronic intoxication. The amount of 

 toxin which is exactly neutralised by one " unit " of the 

 standard antitoxin is known as the L dose. By exact 

 neutralisation is meant absence of any reaction, general 01 

 local, at the seat of inoculation, in the inoculated guinea- 

 pig. If toxin broth were a single substance, containing 

 only toxin, then L, - L = D, the simple lethal dose, 



