The effect of the mast cell growth factor (MGF), also known as stem cell factor, steel factor, and kit ligand, alone or in combination with other GFs on clonogenic blast cell growth in 23 patients with acute myeloblastic leukemia (AML) was investigated. MGF alone enhanced colony formation by about 35%, being clearly stimulatory (> 20% increase in colony numbers) in nine patients. The additive effect of MGF on colony growth was observed in combination with interleukin-3 (IL-3). Preincubation of the cells with MGF in suspension did not sensitize them to the effect of IL-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, or IL-4 in a clonogenic cell culture assay. Although almost all the blast cell samples expressed the c-kit the receptor for MGF, at the mRNA and/or the protein level, the cells did not necessarily respond to exogenous MGF. On the other hand, blast cells were able to respond to exogenous MGF even when the cells themselves expressed MGF. Neither the expression of MGF nor the response of blast cells to exogenous MGF was related to the capability of the cells to form colonies spontaneously. In conclusion, MGF alone, but especially combined with IL-3, was a potent growth factor for clonogenic blast cells in AML. Autocrine production of MGF by AML blast cells analyzed at the mRNA level was not related to autonomous growth of the cells.