The mast cell growth factor (MGF) affects migration, proliferation and differentiation of erythroid and myeloid progenitor cells by binding to a transmembrane receptor tyrosine kinase encoded by the c-Kit proto-oncogene. By using MGF-dependent human myeloid cell lines (M-07e and TF-1), here we show that a Kit-related 100 kDa protein is associated with the cell but it undergoes release into the medium upon treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C. Immunological analysis with a series of antibodies to Kit indicated that the released protein (p100Kit) contains the whole glycosylated extracellular portion of the transmembrane Kit protein (p145Kit). The secreted protein retained the ability to specifically bind MGF. Moreover, p100Kit was able to block the mitogenic effect of MGF on cultured M-07e cells, suggesting that the soluble protein may function as a physiological antagonist of MGF.