Stromal cell lines, designated MS-1, -2, -3, -4, -5, -6, and -7 were established by irradiating the adherent cells in long-term bone marrow cultures with 900-rad x-rays. Two of the cell lines, MS-1 and MS-5, have the capacity to support the growth of hemopoietic stem cells (spleen colony-forming cells and granulocyte-macrophage colony-forming cells) for greater than 2 months in vitro. These two cell lines were alkaline phosphatase-, peroxidase-, and factor VIII-negative and positive for periodic acid-Schiff and nonspecific esterase. Extracellular matrix proteins such as fibronectin, laminin, and collagen type I were produced by these two cell lines. Neither MS-1 cell- nor MS-5 cell-conditioned medium supported the growth of hemopoietic stem cells, and hemopoietic stem cells were found preferentially to be under and on MS-1 and MS-5 layers rather than in suspension. Close contact with the MS-1 cell layer or the MS-5 cell layer appears to be essential in maintaining hemopoiesis in vitro. Conditioned media from MS-1 cells and MS-5 cells stimulated granulocyte colony formation from murine bone marrow cells in semisolid culture.