• Neutropenia commonly occurs in neonates with necrotizing enterocolitis (NEC). In an attempt to determine the etiology of this neutropenia, we observed the peripheral and peritoneal neutrophil cell responses of seven infants at the time of surgery. Six of seven patients had diminished peripheral neutrophil counts within 24 hours prior to surgery, with substantial mobilization of mature neutrophils into the peritoneum. We also looked at the rat peritoneum as a model for neutrophilic cell consumption using casein and Vibrio cholerae enterotoxin to cause neutrophil mobilization. With both agents, significant mobilization of neutrophils into the peritoneum occurred. Bone marrow stores in the animals were substantially decreased, but neutropenia was not observed. We speculate that the neutropenia of NEC is largely a consequence of neutrophil mobilization into the peritoneum, perhaps initiated by dietary protein and/or bacterial toxin.