The effect of stem cell factor (SCF) on the establishment of hematopoietic activity in murine long-term bone marrow cultures (LTBMC) was investigated by addition of SCF to (a) normal LTBMC from the onset of culture and (b) pre-established irradiated bone marrow stroma inoculated with lineage negative (Lin-) primitive hematopoietic progenitor cells enriched on the basis of low rhodamine-123 uptake (Rh-dull). Hematopoietic activity was established more rapidly in LTBMC grown in the presence of SCF (70 ng/mL), and the typical decline in cellularity and progenitor cell content during the first weeks of culture was not observed. SCF also promoted the rapid expansion of progenitor cells derived from Lin-, Rh-dull primitive hematopoietic cells inoculated onto irradiated preestablished bone marrow stroma. The data demonstrate that exogenous SCF augments hematopoietic activity in LTBMC, and that the levels of endogenous SCF elaborated in LTBMC may be suboptimal for expansion of hematopoietic cells.