Mice carrying certain mutations in the white spotting (W) locus (ie, c-kit) exhibit reduced c-kit tyrosine kinase-dependent signaling that results in mast cell deficiency and other phenotypic abnormalities. The c-kit mutations in Kit^W/W-v mice impair melanogenesis and result in anemia, sterility, and markedly reduced levels of tissue mast cells. In contrast, Kit^W-sh/W-sh mice, bearing the W-sash (W^sh) inversion mutation, have mast cell deficiency but lack anemia and sterility. We report that adult Kit^W-sh/W-sh mice had a profound deficiency in mast cells in all tissues examined but normal levels of major classes of other differentiated hematopoietic and lymphoid cells. Unlike Kit^W/W-v mice, Kit^W-sh/W-sh mice had normal numbers of TCRγδ intraepithelial lymphocytes in the intestines and did not exhibit a high incidence of idiopathic dermatitis, ulcers, or squamous papillomas of the stomach, but like Kit^W/W-v mice, they lacked interstitial cells of Cajal in the gut and exhibited bile reflux into the stomach. Systemic or local reconstitution of mast cell populations was achieved in nonirradiated adult Kit^W-sh/W-sh mice by intravenous, intraperitoneal, or intradermal injection of wild-type bone marrow-derived cultured mast cells but not by transplantation of wild-type bone marrow cells. Thus, Kit^W-sh/W-sh mice represent a useful model for mast cell research, especially for analyzing mast cell function in vivo.