<prompt> One unit of diphtheria antitoxin was defined as the smallest amount of antitoxin required to neutralise ?100 MLD of toxin200 MLD of toxin300 MLD of toxin400 MLD of toxin  <response>: Ans. is a i.e., 100 MLD of toxin . Minimum lethal dose MLD of the diphtheria toxin is defined as the least amount of the toxin required to kill a guinea pig weighing 250 gm within 96 hours after subcutaneous inoculation. . One unit of antitoxin was defined as the smallest amount of antitoxin required to neutralize 100 MLD of toxin. 