1. Normal rabbits injected intravenously with diphtheria antitoxin obtained from a horse vary considerably both in regard to the duration of Phase B and the duration and the intensity of Phase C. 2. Phase C, the phase of accelerated loss due to formation of precipitin appears sooner and is more pronounced in older rabbits. 3. The early appearance and more rapid formation of antibody (precipitin) in certain rabbits is indicative of naturally acquired immunity. 4. The highest concentration in the blood of rabbits injected subcutaneously with antitoxic horse serum is seen after 2 to 3 days, when it reaches approximately the same value as that of rabbits injected intravenously, and represents only one-third of the total antitoxin injected. Two hours after subcutaneous injection the antitoxic content is only one two-hundredth of that of a rabbit injected intravenously.