The human stromal fibroblastoid cell strain designated ST-1 represents a normal population of cells capable of supporting hematopoiesis in vitro. These cells constitutively elaborate hematopoietic growth factor activity into the medium and the level of production of this activity dramatically increases following stimulation of the cells with IL-1. This enhanced production is due at least in part to increased expression of the genes for GM-CSF, G-CSF, and IL-6, but not IL-3. The IL-1 treatment had little effect on the expression of M-CSF, a factor made constitutively by the cells. These results are consistent with the model that hematopoiesis is regulated at least in part by constant short-range interactions of humoral factors produced by stromal cells both with other types of stromal cells and with the hematopoietic progenitors.