The indigenous intestinal flora and an intact mucosa are vital components of body defences against luminal pathogenic bacteria. Disruption of these defences in inflammatory bowel disease may permit bacterial translocation and contribute to disease severity. Support for this hypothesis comes from this study of a hapten-induced rat model of colitis. Induction of colitis was associated with a significantly increased colonic Gram-negative aerobic bacilli count. The results, expressed as log10 [colony-forming units per gram tissue] were: colitic 6.97-8.86 versus control 4.90-6.69 (P < 0.05). Colitis was also associated with a decreased Gram-positive cocci count at 4.00-8.04 versus control 6.45-8.30 (P < 0.05). Bacteria translocated to the mesenteric lymph nodes in five of eight colitic rats (P = 0.01), to the spleen in four (P = 0.04) and to the liver in five (P = 0.01) but to these organs in none of the eight control animals. There was a positive correlation between the severity of colonic inflammation and extent of bacterial translocation in colitic animals (rs = 0.86, P = 0.007).