A thymic stromal cell line with a medullary phenotype (Z210R. 1) supported the differentiation of surface IgM+ B cells when cocultured with fetal liver cells in vitro. Conditioned medium (CM) from this cell line supported the long-term growth of a B cell line (NAG8/7) isolated from cocultures and enhanced the proliferation of unfractionated thymocytes to suboptimal concentrations of anti-CD3 antibodies in vitro. Biological assays of the CM detected interleukin-7 (IL-7) but not IL-1, IL-2, IL-3, IL-4, IL-6, Steel factor (SCF), leukemia inhibitory factor (LIF), or macrophage or granulocyte colony-stimulating factors (M-CSF or G-CSF). The failure of recombinant IL-7 to maintain the long-term growth of NAG8/7 cells and the inability of anti-IL-7 antibodies to significantly affect the response of either NAG8/7 cells or thymocytes to CM suggested the presence of one or more other cytokines in the CM. Analysis of concentrated CM fractionated by anion exchange chromatography revealed a single peak of activity in the NAG8/7 assay with an elution profile that was distinct from IL-7. Two peaks of activity were detected in the thymocyte response to anti-CD3 antibodies; one corresponded to IL-7 and the other corresponded to the same fractions that stimulated NAG8/7 cells. The second peak of thymocyte stimulatory activity could not be inhibited by neutralizing anti-IL-7 antibodies. In addition to producing a cytokine with unique properties, this thymic stromal cell exhibits a functional homology to bone marrow or fetal liver stromal cells not previously appreciated.