Mice infected intraperitoneally (i.p.) with Yersinia enterocolitica developed an inflammatory response, as revealed by a large influx of leukocytes in the peritoneal cavity. When the infection was preceded by the administration of Y. enterocolitica by the same route 4 days before, this resulted in a poor inflammatory reaction. On the other hand, the response of previously immunized animals to infection resembled to those of primoinfected mice. Bone marrow cellularity was decreased after the infection with Y. enterocolitica. Since bone marrow depletion by pre‐treatment with cyclophosphamide decreased the inflammatory response to Y. enterocolitica, we concluded that marrow cell reserve was necessary for the inflammatory reaction, whereas specific immunity did not affect this response.