A mechanism to control development of mast cells was investigated in mice. Although mast cells in the skin of normal C57BL/6 mice were still of host type 290 days after irradiation and injection of bone marrow cells from beige (Chediak-Higashi syndrome, C57BL/6 bgJ/bgJ) mice, donor-type mast cells with giant granules appeared after painting of methylcholanthrene on the dorsal skin. Since donor-type mast cells appeared only at the painted portion of the skin, with an increase in the labeling index of such donor-type mast cells with 3H-thymidine, proliferation and differentiation of bone marrow-derived precursors of mast cells seem to be controlled locally. Although the morphologic feature of marrow-derived precursors was not identified, the finding that all fibroblasts cultured from the methylcholanthrene-treated skin were of host type may exclude the possibility that fibroblasts are the precursors of mast cells.

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