Recombinant rat stem cell factor (SCF) was studied for its ability to stimulate the growth of murine hematopoietic progenitor cells and to generate colony-forming cells (CFC) from highly enriched populations of hematopoietic cells. In serum-deprived cultures, SCF alone stimulated few colonies but interacted with a number of other hematopoietic growth factors, particularly interleukin 3, to promote colony formation. The most marked effect was on the generation of mixed-cell colonies. Hematopoietic cells were sorted into wheat-germ agglutinin-negative, monocyte-depleted, rhodamine 123 (Rh123)-bright or Rh123-dull cells. Historically, Rh123-bright cells are capable of short-term (less than 1 mo) marrow engraftment, whereas among Rh123-dull cells are cells capable of long-term marrow engraftment. Enriched cells (2.5 x 10(3) were placed into serum-deprived liquid cultures with various hematopoietic growth factors. Initially, the Rh123-bright and Rh123-dull cells had few CFC but, in the presence of interleukin 3 and SCF, Rh123-bright cells gave rise to greater than 15,000 granulocyte/macrophage CFC, greater than 1500 erythroid burst-forming cells, and greater than 700 mixed-cell CFC by day 5. In contrast, Rh123-dull cells proliferated only in the presence of interleukin 3 and SCF, but total cell numbers rose to a peak of 18,000 by day 21, and one-third of the cells were CFC. Thus, SCF, in combination with other growth factors, can generate large numbers of CFC from pre-CFC and appears to act earlier than hematopoietic growth factors described to date.