AIMS--Animal studies have shown that antigens present within the gut play an important role in the development of acute graft versus host disease (GvHD) following allogeneic bone marrow transplantation (BMT). In previous studies, inert sugars have been found to penetrate the small bowel mucosa after conditioning therapy for BMT; endotoxaemia can also occur during acute GvHD. Data on absorption of antigenic proteins across the gut following BMT in humans have not been presented as yet. METHODS--Six patients undergoing allogeneic BMT were studied to determine whether enteric ovalbumin absorption increased or endotoxaemia developed during acute GvHD. RESULTS--Three patients had minimal antigenaemia and no detectable endotoxaemia before receiving conditioning therapy. At the onset of acute GvHD, however, much higher ovalbumin concentrations were detected in those patients with severe antigenaemia. Serum concentrations of specific antiovalbumin IgG and IgA, or antiendotoxin IgM or IgG had no bearing on detectable IgG or IgM ovalbumin or endotoxin concentrations. In five of six patients, small bowel permeability increased, as tested by the lactulose/mannitol sugar absorption test, but detectable ovalbumin absorption increased in only three of these and only two developed endotoxaemia. CONCLUSIONS--Antigens present within the gut can cross the mucosal epithelium during acute GvHD, probably resulting in an enhanced immune response.