One of the fundamental features of Ehrlich's method of standardising diphtheria antitoxic serum is the recognition of the so-called Lo and L+ limits, the L+ dose being the quantity of a toxin which when mixed with one unit of antitoxin and injected subcutaneously into a guinea-pig leaves sufficient excess of toxin free to cause the death of the animal about the fifth day after injection, the Lo dose being the largest quantity which when similarly mixed with one unit of antitoxin fails to produce a local reaction. In the region between the two doses, the so-called “ Differential-Region ” of Ehrlich, one can obtain a series of mixtures which, while they kill the guinea-pig later than the fifth day or not at all, cause local reactions of varying severity increasing as the L+ limit is reached.