Lr dose the limes reacting dose of diphtheria toxin, i.e., the smallest amount of toxin that, when mixed with one unit of antitoxin and injected intracutaneously in the shaved skin of a susceptible guinea pig, yields a minimal, positive reaction and inflammation localized to the region of the injection; the Lrd. closely approximates the Lod., as would be expected, inasmuch as a slight excess of unneutralized toxin results in a reaction.