Stem cell factor (SCF) is the most important cytokine regulating human mast cell growth and functions. The immunogold technique showed SCF in the secretory granules of skin mast cells and in lung parenchymal mast cells (HLMC). Immunoreactive SCF (iSCF) was detected in cell lysates of HLMC, but not in basophils; iSCF and histamine were detected in supernatants of HLMC 3 min after challenge with anti-FcεRI or anti-IgE, and iSCF in supernatants rapidly declined after 30 min, whereas histamine remained unchanged for 120 min. HPLC and electrospray mass spectrometry (ES/MS) analysis of recombinant human SCF 1–166 (18,656.9±0.9 Da) treated with chymase showed a polypeptide of 17,977.1±0.6 Da and a minor component of 697.4±0.1 Da generated by specific cleavage at Phe159. SCF 1–166 and SCF 1–159 similarly activated HLMC, potentiated anti-IgE-induced activation of these cells, and stimulated HLMC chemotaxis. SCF 159–166 had no effect on mast cells. Western blot analysis of supernatants of anti-IgE-activated HLMC incubated with recombinant human SCF 1–166 showed that SCF 1–166 was rapidly cleaved to SCF 1–159 and SCF 1–144. Experiments with supernatants of anti-IgE-activated HLMC incubated with SCF 1–166 yielded similar results. In conclusion, SCF is stored in mast cell secretory granules and is immunologically released by human mast cells. SCF 1–166 is rapidly and specifically cleaved to SCF 1–159 by chymase, which retains its biological effect on mast cells. SCF is also cleaved by other proteases to several SCF species whose possible biological activities remain to be established.