The hemopoietic inductive microenvironment (HIM) of the bone marrow is responsible for secretion of growth factors that regulate hemopoiesis. It is composed of an extracellular matrix and a complex variety of cell types with a range of functions related to blood cell development. In order to understand how such a complex system operates, it will first be necessary to determine the role(s) of the integral parts. Several of the stromal cell types have been identified morphologically in various culture systems, and some of their functions have been elucidated. We have identified a new stromal cell type in mouse bone marrow that appears similar if not identical to its human counterpart. When bone marrow cells are placed in methylcellulose/plasma clot culture with phytohemagglutinin-stimulated human leukocyte-conditioned medium in the presence of bovine calf serum (BCS), mercaptoethanol, and hydrocortisone, extensive branching colonies develop within 14 days. These "reticulo-fibroblastoid" (RF) colonies arise from a putative reticulo-fibroblastoid colony-forming unit (CFU-RF) stem cell, and many become adipocytic by day 14; the addition of fresh medium, methylcellulose, and BCS on day 7 inhibits this change. The batch of human citrated plasma used in the culture system and the type and source of stimulating factor all influence the number of RF colonies that develop as well as the percent of colonies that become adipocytic. Whether this adipogenesis represents functional maturity or terminal differentiation is not yet known. Information gained on the role of these RF cells in normal and impaired hemopoiesis should contribute to the elucidation of the complicated interactive role of the microenvironment in the support and modulation of hemopoiesis.