The response of marrow stromal cells transformed with wild-type simian virus 40 to recombinant growth factors was examined. When transformed stromal cells were plated in semisolid medium without the addition of growth factors, only 0.4% of cells formed colonies while with the addition of recombinant factors such as interleukin 1 (IL-1) or tumor necrosis factor (TNF), up to 10% of the cells formed colonies. Colonies were individually plucked and cell lines were developed that could be analyzed for expression of growth factors. The data show that unstimulated marrow stromal cells lines produced no detectable colony-stimulating activity. However, cell lines derived from "autonomously growing colonies" and from colonies grown with T cell-conditioned medium, with IL-1 alpha or beta, or with TNF alpha produced colony-stimulating activity and transcripts for granulocyte/macrophage-colony-stimulating factor (CSF), granulocyte-CSF, and IL-1 beta. A novel feature of the cell lines derived from colonies was that the production of growth factors was constitutive and persisted in excess of 4 m.