The functional relationships between bone marrow fibroblastoid cells, granulocyte-macrophage colony-forming cells (GM-CFC) and colony-stimulating factor (CSF) have been investigated in vitro. Cultured fibroblastoid cells neither produce CSF nor release a factor which increases the production of CSF by plastic-adherent colony-stimulating cells (CSC). However, the presence of fibroblastoid cells in agar cultures containing performed CSF increases the plating efficiency of GM-CFC. The fibroblastoid cells seem to act either by increasing the stimulatory activity of CSF or by releasing a stimulatory factor in the presence of CSF. The sensitivity of the colony-forming cells to CSF was not modified by contact with the fibroblastoid stromal cells. Fibroblastoid cells themselves were required for demonstration of the enhancement of CSF since the effect could not be reproduced using supernatant medium from fibroblast cultures. Different effects were seen when supernatant media from fibroblastoid cultures were incorporated into agar cultures of GM-CFC or into liquid cultures of bone marrow cells. The culture supernatants contain soluble factors which inhibit the formation of granulocyte-macrophage colonies in CSF-stimulated agar cultures and activities which increase the survival of GM-CFC in liquid cultures. The results suggest that stable long-range and transient short-range influences of bone marrow fibroblastoid cells may be involved in the regulation of granulopoiesis.