The effect of activated-lymphocyte supernatant on glia cells was investigated. When treated in vitro with Concanavalin A (ConA), murine spleen cells released a soluble product, termed glia cell stimulating factor (GSF), which stimulated RNA and DNA synthesis in cultured murine glia cells. Furthermore, GSF appeared to promote the maturation of undifferentiated glia cells to astrocytes having a high content of glial fibrillary acidic protein. GSF secretion occurred after a lag period of 16 hours and proceeded at a constant rate for more than 48 hours. This GSF produced by ConA-stimulated murine lymphocytes has an apparent molecular weight between 60,000 and 80,000. Antigenic stimulation of primed lymph node cells with BGG resulted in a similar GSF production. Cellular sources of mitogen-induced GSF were investigated by using isolated lymphoid populations. GSF release by ConA-activated pure T-lymphocytes reconstituted with peritoneal macrophages was equivalent to that of unseparated spleen cells, whereas GSF production by T-lymphocytes alone was low. Macrophages alone did not elaborate detectable levels of GSF. GSF was also secreted by enriched -B-lymphocytes populations stimulated by Protein A. Formation of GSF was suppressed when cytochalasin B or cyclo-heximide was added to the cultures, while colchicine failed to have any effect. DNA synthesis is not required for GSF production as determined by resistence to treatment with mitomycin C. The data indicate that the GSF production and secretion mechanism is much like that described for other lymphokines.