Recombinant human stem cell factor (rhSCF) induced histamine release from human skin and lung mast cells but had no effect on human basophil leukocytes. More importantly, rhSCF enhanced the release of histamine in response to IgE-crosslinking of human mast cells. This potentiation was observed at rhSCF concentrations which induced histamine release, and also at lower concentrations of the ligand which by themselves produced no effect. Very limited potentiation was observed with human basophil leukocytes. The enhancing effect of SCF was unique to IgE-dependent stimulation and when SCF was incubated with the neurotransmitter substance P and the calcium ionophore A23187, no augmentation of histamine release was observed with any of the cell types tested. These findings suggest that endogenous SCF may contribute to the regulation of mast cell function and may hence play a role in diverse allergic and inflammatory reactions.