Interleukin-3 (IL-3)-dependent murine FDCP-mix cells have multilineage differentiation capacity; they are nonleukemic, have a normal karyotype, and are nonimmortalized. These cells coexpress on their cell surface the “early” B-lineage marker B220/CD45R and the myeloid marker Mac-1/iC3b receptor (CR3), transcribe germline T-cell receptor gamma genes, and express the macrophage lineage growth factor receptor gene c- fms as a predominant 8.4-kb transcript. They do not detectably express at the stable mRNA or protein level other lymphoid precursor cell genes including CD2, TdT, lambda 5, and BP1. Induction of granulocyte/macrophage differentiation in these cells closes down expression of the lymphoid genes and activates stable expression of genes specific to the myeloid lineage, including myeloperoxidase. Expression of the c-fms gene at the mRNA level is upregulated and the dominant stable transcript is now in the 4.1-kb form typical of the macrophage lineage. These data provide a plausible explanation for the coexpression of lymphoid and myeloid lineage markers on human leukemic cells of stem cell or progenitor cell origin and have implications for the programming of lineage potential in normal multipotential hematopoiteic progenitor cells.