Background: P-glycoprotein (P-gp), the product of the multidrug resistance gene (MDR), is an ATP-dependent transmembrane pump, which is expressed in multiple cell lineages including epithelial and hematopoetic cells. The human MDR gene is located on chromosome 7 (7q21.1), a susceptibility loci for inflammatory bowel disease (IBD). A significant number of IBD patients carry mutations in this gene and P-gp-deficient FVB/N mice develop a severe spontaneous colitis, characterized by impaired intestinal barrier function and immune reactivity to intestinal bacterial antigens. Methods: In this work we explored the role of mouse strain, as well as environmental insults, on the development of colonic inflammation in the absence of P-gp. Among the induction methods utilized, dextran sodium sulfate (DSS) disrupts the intestinal epithelium, while piroxicam is a nonsteroidal antiinflammatory (NSAID) drug that inhibits prostaglandin production and initiates colitis in IL10-deficient animals. Helicobacter bilis is a known mediator of bacterial-induced colitis. Results: We demonstrate that crossing this mutation onto the C57BL/6 strain confers protection from spontaneous colitis. C57BL/ 6.mdr1α-deficient animals demonstrated increased histological inflammation, colonic shortening, fecal blood, and reduced body weight after 7 days of treatment with 2.25% DSS. C57BL/6.mdr1adeficient mice treated with piroxicam or infected with H. bilis showed no weight loss, or alterations in colonic histology. Conclusions: These data indicate that the effects of P-gp deficiency are significantly modulated by background strain influences, but that the epithelium continues to have increased susceptibility to chemical injury in the C57BL/6 model. Copyright © 2009 Crohn's & Colitis Foundation of America, Inc.