By using pigs sensitized to bovine serum albumin (BSA), it was found that exposure of the intestinal mucosa to BSA induced, in 4 h, the emigration of large numbers of neutrophils into the intestinal lumen. This response was specific for the immunizing antigen and could be transferred to nonimmune animals with immune serum. The emigration of neutrophils through the intestinal mucosa was not accompanied by the edema, hemorrhage, and thrombosis which were apparent after intracutaneous inoculation of BSA into the same animals. Twenty-four hours after a 4-h mucosal exposure to BSA, the intestinal mucosa showed no evidence of neutrophil emigration nor any other abnormal features. These observations suggested that emigration of neutrophils into the intestinal lumen can be a specific, antibody-mediated immune response which occurs in the absence of intestinal injury. Possible relationships between immune-mediated enteroluminal emigration of neutrophils, neutrophil production, and a protective role for the neutrophil in the intestinal lumen were considered.