Fibroblasts from a variety of tissues interact with and influence the behavior of the cell types they are associated with by producing specific proteins that mediate these interactions. Thus, it is not surprising that fibroblasts have been shown to differ phenotypically and functionally depending on the tissue they are isolated from and its physiologic state. To study fibroblasts of hematopoietic tissues, cultures were established from human normal bone marrow (BM), and from non-myelometaplasic (NS) and myelometaplasic spleen (MMS) tissues and analyzed for phenotypic characteristics. The results are summarized as follows: (1) cytoskeletal elements: virtually all the MMS fibroblasts were stained positively for alpha-sm-actin while only a small fraction of BM and of NS fibroblasts were positive for this antigen; (2) extracellular matrix elements: MMS fibroblasts stained positively for ED-B fibronectin and tenascin while the other 2 fibroblast cell types did not; (3) cell surface molecules: NS and MMS fibroblasts expressed significantly higher levels of ICAM-1, VLA-4 and CD9 than BM fibroblasts. Moreover, MMS fibroblasts showed a higher expression of ICAM-1 and VLA-4 than NS fibroblasts; and (4) cytokines: IL-II, RANTES and MIP-1alpha were produced in higher amounts by BM than by NS fibroblasts. Conversely, production of GM-CSF, SCF, M-CSF and MCP-1alpha was elevated in NS compared with BM fibroblasts. The production of these cytokines was generally reduced in MMS cells. Overall, our results demonstrate that phenotypic characteristics can be identified to distinguish fibroblasts from normal and pathologic hematopoietic tissues. Such phenotypic characteristics suggest functional differences of each type of fibroblast in their influence on the blood cells with which they are associated.