Intestinal mucosal barrier function in obstructive jaundice was assessed in an animal model and in patients. The effect of internal biliary drainage in patients was also examined. Bile duct ligation for 1 week in the rat resulted in significant bacterial translocation (in seven of 12 animals following ligation versus none of the shamoperated controls, P < 0.01). Intestinal permeability, measured by the urinary recovery of orally administered polyethylene glycol, was also significantly increased (+66.2 per cent for ligation versus -11.6 per cent for sham, P < 0.01). A prospective study was performed on 33 patients with obstructive jaundice undergoing internal biliary drainage, and results were compared with those in six non-jaundiced patients undergoing laparotomy or endoscopic retrograde cholangiopancreatography and in 11 health volunteers. The lactulose: mannitol ratio was used as an intestinal permeability index. Mean(s.e.m.) intestinal permeability assessed before operation was significantly increased in jaundiced patients compared with control patients (0.050(0.010) versus 0.016(0.003), P < 0.005). The mean(s.e.m.) lactulose: mannitol ratio in the healthy volunteers was 0.020(0.003), which was similar to that in control patients. In the jaundiced group of patients the intestinal permeability index fell to within normal levels after 28 days of internal biliary drainage (0.050 before operation versus 0.021 at 28 days, P < 0.02). These data indicate that intestinal barrier function is impaired in obstructive jaundice and that this impairment is reversed by return of bile to the gastrointestinal tract.