Kit, the receptor for stem cell factor (SCF) and the product of the c-kit proto-oncogene, is expressed on fetal liver-derived mast cell progenitors when cultured with SCF. Decreased levels of Kit on the surface of human fetal liver-derived mast cells after exposure to recombinant human SCF were demonstrated by flow cytometry using the YB5.B8 mAb against Kit. Internalization of Kit along with SCF appears to be the principal means by which Kit is lost from the mast cell surface. Neither the beta 3-integrin CD51/CD61 (alpha v beta 3), nor the beta 1-integrins CD49d,e/CD29 (VLA-4 and -5) appeared to be internalized along with Kit-SCF complexes. Reappearance of Kit on day 28 fetal liver-derived mast cells is complete 3 days after exposure of the cells to SCF and is detectable by 2 days. Recovery requires new protein and new RNA synthesis, because Kit did not reappear if cycloheximide or actinomycin D was added to the cells. No substantial change in total Kit mRNA was detected during the resynthesis period, suggesting that post-transcriptional regulation of Kit production is involved. Internalization of Kit in mast cells exposed to soluble SCF may represent a negative regulatory mechanism for this receptor-ligand interaction and down-regulate mast cell properties such as degranulation to SCF.