OBJECTIVE: P-glycoprotein (P-gp), the functional product of the multidrug resistance gene (MDR), is a transmembrane protein that extrudes substrates from the intracellular environment. P-gp is expressed on the apical surface of epithelial cells and on cells from the hematopoietic lineage. Human MDR polymorphisms have been associated with the increased risk of inflammatory bowel disease, and FVB/N animals deficient in mdr1a expression develop spontaneous colitis. Previous studies using adult bone marrow chimeras indicated that colitis development in this animal model was contingent on P-gp deficiency in radiation-resistant epithelial cells; however, the use of adult animals may mask the role of hematopoietic immune cells in colitis initiation, due to preexisting epithelial abnormalities. SUBJECTS AND METHODS: To assess the importance of P-gp expression in intestinal epithelial and hematopoietic-derived cells on colitis induction in FVB.mdr1a -/- animals, we developed a neonatal model of bone marrow reconstitution. FVB/N and FVB.mdr1a -/- adult and neonatal animals were lethally irradiated and reconstituted with bone marrow from FVB/N or FVB.mdr1a -/- donors. Animals were observed for 20 weeks. RESULTS: Adult FVB/N animals deficient in P-gp expression in hematopoietically derived immune cells developed colitis similar to adult animals deficient in P-gp expression in radiation-resistant epithelial/stromal cells. Neonatal animals deficient in P-gp expression in hematopoietically derived immune cells developed a more histologically significant colitis than those deficient in P-gp expression in epithelial tissue. CONCLUSIONS: The use of a neonatal model of bone marrow reconstitution has revealed a critical role for P-gp expression in hematopoietically derived immune cells in colitis development in the FVB.mdr1a -/- model. Copyright © 2011 by European Society for Pediatric Gastroenterology, Hepatology, and Nutrition and North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition.