Ehrlich phenomenon is seen in

The difference between the amount of diphtheria toxin that will exactly neutralize one unit of antitoxin and that which, added to one unit 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). Reference: Textbook of Microbiology; Anathanarayan and panicker's; 9th edition