We have performed an immunohistochemical study of intact adherent layers of human long-term bone marrow cultures (hLTBMC) in order to characterize the cell types present. Our panel of antibodies was selected to demonstrate various mesenchymal and haemopoietically derived cell types and to assess the presence of molecules of potential importance as adhesive ligands between haemopoietic cells and stroma. Subpopulations of fibroblasts and macrophages were identified which differed in immunophenotype. We were able to demonstrate modulation of fibroblast and extra-cellular matrix immunophenotypes between 2 and 12 weeks in culture. Stromal cells and matrix expressed a wide variety of antigens of potential importance in haemopoietic cell adhesion, but no ICAM-1, 2 or 3 could be demonstrated to correspond to the strong LFA-1 expression seen in haemopoietic precursor cells. No localization of antigen expression by stromal elements was found to account for the formation of haemopoietic foci at particular sites. However, granulocyte-predominant foci preferentially occupied the interstices and margins of structures which appeared to be vascular arrays.