1. The injection of a single large dose of normal horse serum into rabbits results in the appearance 3 to 8 days later of erythematous and edematous reactions on the ears in 68.9 per cent of the animals.
2. The injections may be given by any of several routes and reactions appear when the site of injection is definitely distant from the ears.
3. Injections of various antisera into rabbits cause the appearance of similar reactions.
4. These reactions can be considered as manifestations of serum sickness in rabbits.