Cell adhesion molecules (CAMs) play a key role in interactions between stromal and hematopoietic cells in bone marrow (BM) and in cell traffic through vascular endothelium. To examine the identity of CAMs involved in these processes in mouse BM, we have investigated the in vivo expression of vascular cell adhesion molecule-1 (VCAM-1) and its counter-receptor, very late antigen-4 (VLA-4). Radioiodinated monoclonal antibodies (MoAbs) detecting VLA-4 and VCAM-1 were injected intravenously. Antibody binding was detected in BM by light and electron microscope radioautography. VCAM-1 labeling was restricted to stromal reticular cells and endothelial cells lining BM sinusoids. VCAM- 1+ reticular cells formed patchy concentrations, especially in subosteal regions, associated with lymphoid, granulocytic, and erythroid cells. After gamma-irradiation to deplete hematopoietic cells, reticular cells and endothelial cells all showed VCAM-1 labeling in apparently increased intensity. VLA-4 labeling was shown by undifferentiated blast cells and lymphohematopoietic cells both in BM cell suspensions and in vivo, especially at reticular cell contact points. The results demonstrate that VCAM-1 is expressed in vivo by certain BM reticular cells, suggesting that the molecule mediates adhesion to multiple lineages of lymphohematopoietic cells. The finding that VCAM-1 is also expressed constitutively by BM sinusoidal endothelium, unlike its inductive expression by endothelia elsewhere, suggests that VCAM-1 and VLA-4 may be involved in regulating the normal cell traffic between BM and the blood stream.