We describe the effect of soluble c-kit ligand (stem cell factor, SCF) on highly purified CD34-positive hemopoietic progenitors from human umbilical cord blood. Progenitor cells were purified from cord blood mononuclear cells by immune rosetting with lineage specific antibodies and subsequent sorting of the rosette-negative population for CD34(BI3C5)-positive cells. This procedure enriched greater than 100-fold for colony forming cells (CFC). Using optimal concentrations of colony-stimulating factors (CSF) without added SCF approximately 2.5% of cells formed colonies. SCF also had CSF activity on this population, up to 0.5% of cells forming small colonies in response to SCF alone. In contrast, the addition of SCF to optimal concentrations of the other growth factors produced a greater than 10-fold increase in colony number. However, the most notable effect was an approximately 100-fold increase in the number of cells in each colony. Equally striking was the very high proportion (50-80%) of mixed colonies (CFU-MIX). These findings suggest the progenitor cell pool in cord blood is skewed towards very early cells. However, when day 14 colonies formed in response to SCF and other factors were assessed for their re-cloning potential they did not contain significant numbers of CFC, implying that SCF did not support the self-renewal of these CD34 positive cord blood progenitor cells. These findings support a role for SCF as an enhancing factor for hemopoietic progenitor cells but it does not promote self-renewal in these populations.