The factors responsible for the activation of astrocytes surrounding inflammatory brain tissues are unknown. The present study was designed to examine the ability of lymphocytes to produce astrocyte stimulating activity in cell culture. Normal rat lymphocytes stimulated with Concanavalin A, or sensitized lymphocytes, challenged with antigen in vitro, activate cultured rat glia cells by a soluble mediator which we have termed glia stimulating factor (GSF). In undifferentiated glioblasts both RNA synthesis, as measured by [5-3H]uridine uptake, and DNA synthesis, as measured by [6-3H]thymidine uptake, were stimulated by the presence of GSF. Preliminary characterisation showed the GSF to be non-dialysable and heat stable at 56 degrees C for 30 min, but not stable at 80 degrees C for 30 min. To study the effect of this factor on differentiated glia cells, brain cell cultures were treated with dibutyryl cyclic AMP (db-cAMP) which induces a morphologic transformation of glioblasts to multipolar cells that have a characteristic astrocytic appearance. After addition of GSF to db-cAMP treated astrocytes only an increase in RNA synthesis was observed. The significance of this in vitro phenomenon, mediated by a glia stimulating factor, to activation of astrocytes and astrocytic gliosis in human brain diseases is discussed.