Recently, soluble(s) circulating isoforms of intercellular adhesion molecule-1 (sICAM-1) and sE-selectin (formerly endothelial leukocyte adhesion molecule-1) have been described in normal human serum. Elevated levels have been reported in acute and chronic inflammatory disorders, including allograft rejection. In this study, plasma levels of sICAM-1 and sE-selectin were determined in groups of tacrolimus (FK 506)-treated adult patients following either isolated small bowel (SB), liver, or combined SB plus liver (SB/L) transplantation. Each molecule was measured at 1, 2, 4, 6, 8, and 12 weeks (all patients) and at 6, 9, and 12 months after transplantation (SB and SB/L only) by enzyme linked immunosorbent assay. Levels were compared with those of soluble interleukin-2 receptor (sIL-2R; a marker of lymphocyte activation) and soluble HLA class I (which has been reported to be elevated in liver transplant-related complications). Elevations above normal in mean plasma levels of sICAM-1 (2.4-fold), sE-selectin (1.8-fold), sIL-2R (10.6-fold), and sHLA class I (1.3-fold) were found in patients with stable isolated SB grafts during the first 12 weeks posttransplant. Except for sHLA class I, levels of each protein were subsequently reduced, up to 1 year posttransplant. However, further increases in sICAM-1 and in sIL-2R and sE-selectin levels were observed during episodes of SB rejection compared with stable grafts. Mean levels of all molecules were higher in patients with isolated SB grafts compared with those given liver or combined (SB/L) transplants, either during stable SB graft function (up to 12 weeks posttransplant) or rejection. The data demonstrate increased adhesion molecule production/shedding following SB transplantation and are suggestive of a reduced overall level of immune activation in liver and SB/L compared with isolated SB transplantation.