1. There is some evidence that an injection of 0·00001 c.c. of horse serum causes an appreciable degree of active immunity to horse serum in rabbits.

2. Rabbits actively immune to horse serum receiving a series of injections of antitoxic horse serum at intervals of three to ten weeks eliminated the antitoxin at rapidly increasing rates.

3. An intravenous injection of antitoxic horse serum given six weeks after the second of a series of 0·5 c.c. in a rabbit already actively immune to horse serum was eliminated so rapidly owing to excess precipitin that less than 1/1000th remained 24 hours after injection.

4. Another rabbit which, six days after a previous injection, was given

serum intravenously, lost over 99 per cent. of the antitoxin within 15 minutes of the injection.

5. The maximum of circulating antitoxin detected after the second subcutaneous injection into a sensitised rabbit was only 1/15,000th of the amount injected.

6. Antitoxic horse serum injected intravenously into rabbits at weekly intervals was rapidly eliminated and less than 2 per cent. could be detected 24 hours after each injection.

7. After 12 daily intravenous injections of antitoxic horse serum into a rabbit, the antitoxic content was only one-half of that after the first injection; later injections were not eliminated so quickly and precipitin formation gradually ceased.

8. The rate of elimination of antitoxic horse serum in a rabbit may be greatly delayed by daily injections of normal horse serum.