338 A MANUAL OF BACTERIOLOGY 



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 were found 

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

 of standard antitoxin, 0-2, 0-3, 0-4, and 0-5 c.c. respec- 

 tively 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 unneutra- 

 lised, 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, death 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 or 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, would be 

 equal to the minimal lethal dose. But because of the 

 presence of toxone, which also has an affinity for anti- 

 toxin, D, the difference between the L + and the L 



