The cellular mechanism by which PTH and other osteotropic substances stimulate bone resorption is unclear. One hypothesis is that PTH-stimulated osteoblasts release cytokines which activate osteoclasts or osteoclast precursors. To examine whether cytokines are released by osteoblast-like cells in vitro, medium conditioned by a clonal rat osteosarcoma cell line 17/2.8 (ROS) was examined for mitogenic activity using a helper T lymphocyte line HT-2. This line proliferates in response to interleukin-2 (IL-2), IL-4, and granulocyte-macrophage colony-stimulating factor (GM CSF). Conditioned medium (CM) from untreated ROS cells caused proliferation of HT-2 cells. Treatment of ROS cells with PTH or lipopolysaccharide (LPS) caused a dose-dependent increase in the secretion of this mitogenic activity. To further define the nature of this mitogenic activity, we examined the effect of incubation of CM with neutralizing antibodies to IL-2, IL-4, and GM CSF. Mitogenic activity induced by both PTH- and LPS-treated ROS cell CM was completely inhibited by anti-GM CSF antibody, whereas there was no reduction in activity in the presence of antibodies to IL-2 or IL-4. Partial purification of both PTH- and LPS-treated CM using reverse phase HPLC resulted in a single peak of HT-2 mitogenic activity, which in both cases was completely inhibited by anti-GM CSF antibody. These findings suggest that PTH- and LPS-treated ROS cells secrete a T cell mitogenic activity which, by functional, serological, and biochemical criteria, is indistinguishable from GM CSF.