Abstract The regulation of tissue mast cell number depends both on the rate of production of mast cell precursors from bone marrow and the length of survival of mature mast cells within tissues. Mast cells develop from bone marrow under the influence of both interleukin-3 (IL-3) and the c-kit ligand, also known as stem cell factor (SCF). In humans, the mast cell precursor is CD34+, FcERI-. Mast cell precursors with time become less responsive to IL-3 and more responsive to SCF. Mast cell proliferation directed by SCF is enhanced by other cytokines including both IL-4 and IL-10. Once mast cell precursors target to tissues, their survival may largely be dependent upon the local production of SCF. Withdrawal of IL-3 or SCF results in mast cell apoptosis; SCF rescues mast cells following IL-3 withdrawal. TGF-beta prevents this SCF rescue. Engagement of extracellular matrix by integrin receptors may also effect mast cell numbers. Thus, in the final analysis, mast cell numbers, while relatively constant in the normal state, may be up-regulated by altering the rate of their production centrally or length of survival in the periphery.