Expression of integrins, a superfamily of glycoprotein alpha/beta heterodimers which integrate the cytoskeleton with the extracellular matrix and/or mediate cell-cell adhesive interactions, was examined on normal and leukaemic bone marrow cells by immunohistochemistry and immunotransmission electron microscopy (immuno-TEM). Among the beta 1/VLA molecules studied, VLA-2 and 6 were expressed on megakaryocytes and platelets, while VLA-4 was present on 40% of haemopoietic cells, including monocytes, erythroblasts and immature cells; this molecule was typically localized at sites of intercellular contact, as seen by immuno-TEM, suggesting it may be involved in interactions among haemopoietic cells during differentiation. In human long-term bone marrow cultures (LTBMC), VLA-1 and 3 were present respectively on 35% and 40% of the adherent cells which included fibroblasts and endothelial cells, as shown by double-labelling experiments; VLA-2 was expressed only on a subpopulation of fibroblasts. beta 2/LeuCAM molecules were absent from platelets, megakaryocytes and HLA-DR+/myeloperoxidase- early myeloid precursors, and appeared progressively during maturation in both lymphoid and myeloid cells. Expression of beta 3/cytoadhesin molecules was restricted to megakaryocytes and platelets and, in the adherent layer of LTBMC, to endothelial cells. The regulated expression and specific localization of integrins in the bone marrow suggest that these molecules may have a role in normal haemopoiesis.