Rejection seen after small bowel transplantation (SBT) in the rat is associated with an accumulation of hyaluronan (HA) in the lamina propria of the graft. In this study the intestinal intraluminal content of this connective tissue component was measured in order to determine if HA could be used as an index of rejection following SBT. Syngeneically and semiallogeneically transplanted rats were investigated by perfusion of a 4-cm segment of the intestinal graft on days 2, 4, and 6 posttransplantation. The amounts of HA recovered during perfusion were analyzed using a radiometric assay. In rejecting grafts the recovered HA amounts increased 15 times from day 2 to day 6, although after syngeneic transplantation there was only a minor increase from day 2 to day 4 and no further increase on day 6. Thus, the recovery of HA during perfusion of transplanted small bowel grafts has the potential value to reflect rejection.