Helicobacter bilis has been isolated from aged inbred mice with multifocal chronic hepatitis and from scid mice with diarrhea, proliferative typhlitis, and colitis. To determine the pathogenic potential of H. bilis, we inoculated 4-week-old female Tac:Icr:Ha(ICR)-scidfDF mice by intraperitoneal injection of approximately 10(8) CFU of H. bilis in phosphate-buffered saline (PBS) (n = 15) or PBS alone (n = 10) and necropsied them at 7 weeks postinfection. Sham-inoculated mice had no significant gross or histopathological findings. In contrast, all 15 experimentally inoculated mice (confirmed to be H. bilis-colonized by culture and PCR of cecal contents) exhibited varying degrees of inflammatory bowel disease (IBD). Proliferative typhlocolitis was characterized by focal to segmental areas of crypt hyperplasia and a predominantly histiocytic inflammatory cell infiltrate. Labeling indices for 5-bromo-2'-deoxyuridine incorporation were increased approximately 2.5-fold in the ceca and colons of H. bilis-inoculated mice. This is the first study to demonstrate experimentally that infection with H. bilis causes IBD in scid mice with defined flora. This result both confirms a pathogenic role for H. bilis in mice and provides a new model relating a specific microbial agent and IBD.