the difference between the amount of diphtheria toxin that will exactly neutralize 1 U of antitoxin and that which, added to 1 U of antitoxin, will leave one lethal dose free is greater than one lethal dose of toxin; i.e., it is necessary to add more than one lethal dose of toxin to a neutral mixture of toxin and antitoxin to make the mixture lethal (the basis of the L+ dose).