BOTH basophilic granulocytes and tissue mast cells are known to have receptors for immunoglobulin E (IgE) and granules which contain histamine and heparin1,2. In spite of their similar physiological role, these two types of cells are thought to be independent cell lineages; the former are derived from the bone marrow, and the latter are connective tissue cells2. The differentiation of mast cells from their precursors, which were supposed to be undifferentiated mesenchymal cells, and the proliferation of these precursors were reported to occur in the skin of mice3,4. These results do not necessarily exclude the possibility that the precursor cells themselves are originated from the bone marrow, however. To examine such a possibility, we investigated whether mast cells of the donor origin appeared in radiation chimaeras, using the giant granules of beige (Chediak–Higashi syndrome) mouse5 as a quantitative marker for mast cells. We have found that tissue mast cells can be derived from grafted bone marrow cells in irradiated mice.