1. There is a significant inflammatory response to large homogenous-bone transplants in rats. The incidence of this inflammatory response corresponds to the predictable incidence its randomly selected animals.
2. The inflammatory response is intensified in presensitized homogenous preparations in rats, which may represent a delayed-sensitivity or Arthus-like reaction.
3. Evidence is presented, by means of parabiosis, to show that this reaction may be due to a systemically formed circulating antibody. Confirmatory evidence by immunological methods is needed.
4. This histological demonstration of a delayed sensitivity or Arthus-like reactions, using bone transplants as the antigen and showing indirectly the presence of the circulating antibody, suggests that the rejection of homogenous bone transplants in rats may be due to an antigen-antibody reaction indicating a state of active acquired immunity.