The mice orally vaccinated with viable or formalin-killed cells of serovar O3 of Yersinia enterocolitica were resistant to fecal excretions of the organism when intraperitoneally challenged. The OH agglutinins of the orally immunized mice were boosted upon subsequent intraperitoneal challenge. The mice having developed high serum agglutinin titers of 1:320 or more were protected against fecal excretions of the organism upon subsequent intraperitoneal challenge. Upon intragastrical challenge, however, such high agglutinin titers did not protect the mice against the fecal excretion. Duration of fecal excretion was shortened in the mice having received spleen or mesenteric lymph nodes cells derived from immunized mice.