We examined the expression of VCAM-1 and MAdCAM-1 after bone marrow transplantation (BMT). We also examined the influence of α₄β₇ integrin blockade on the homing of cells to the bone marrow and spleen. The expression of VCAM-1 and MAdCAM-1 by endothelial cells in the spleen and bone marrow was examined by immunoelectron microscopy using colloidal gold and was analyzed semiquantitatively. To examine the role of α₄β₇ integrin in donor cells, a homing assay was conducted following α₄β₇ integrin blockade in bone marrow-derived hematopoietic cells or spleen colony cells. Immediately after BMT, the expression of VCAM-1 and MAdCAM1 markedly decreased, but expression recovered significantly between 12 and 24 h after BMT. VCAM-1 recovered more acutely than MAdCAM-1 from 12 h onward. In the group transplanted with anti-α₄β₇ integrin antibody-treated bone marrow cells, the numbers of homing cells in the spleen and bone marrow were significantly decreased in an antibody dose-dependent manner. However, the number of homing cells was not different in either the spleen or bone marrow between anti-α₄β₇ integrin antibody treated and untreated spleen colony cells. It has been reported that α₄β₁ integrin and its receptor VCAM-1 play major roles in the homing of hematopoietic cells to bone marrow. Our study indicates the importance of MAdCAM-1 and its ligand, α₄β₇ integrin, in the homing of bone marrow-derived hematopoietic cells, but not spleen colony-derived cells, to both the spleen and bone marrow.