To investigate the role of the receptor-type tyrosine kinase, c-kif and its ligand, stem cell factor (SCF) in T cell development, we analyzed c-kit (W/W) and SCF (SIISI) deficient mice. We also engrafted wild-type or SCF-deficient fetal thymi onto wild-type recipient mice and analyzed the rate of proliferation by in vivo bromodeoxyuridine labeling. The results show that the most immature thymocyte compartment defined as CD3-CD4-CDB-is significantly reduced in SVSI grafts and W/W thymi compared with wild-typecounterparts. Also, the expansion rate of these immature thymocytes in SllSl graft is reduced by-50%. These experiments provide direct evidence for an important role for c-kit-SCF interactions in expansion of very early thymocytes.