We have recently found that the number of mast cells in the skin of adult W/W^v mice is less than 1% of that observed in congeneic +/+ mice, and that no mast cells are detected in other tissues of W/W^v mice¹. After the transplantation of bone marrow cells from congeneic +/+ mice, the number of mast cells in the skin, stomach, caecum and mesentery of the W/W^v mice increased to levels similar to those of the +/+ mice¹. Study of the mast-cell number in the W/W^v mice at various times after transplantation suggested to us that mast cells might develop in groups, particularly in the skin and mesentery. In this report, we have attempted to elucidate the possible clonal origin of such mast-cell clusters from a single precursor cell, using giant granules of beige (CS7BL/6-bg^J/bg^J, Chediak–Higashi syndrome) mice as a marker to identify the origin of the mast cells^2–4 (Fig. 1). We found that when WB-W/+×C57BL/6-W^v (WBB6F₁)-W/W^v mice were injected with a mixture of bone marrow cells from beige C57BL/6 mice and normal C57BL/6 mice, more than 95% of mast-cell clusters consisted of either beige-type cells alone or normal-type cells alone. We conclude, therefore, that the cluster of mast cells originated from a single precursor cell.