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Clonal assay of mast cell progenitors in semi-solid culture with a high seeding efficiency.

In vitro clonal assay of murine mast cell progenitors was examined in semi-solid culture by using conditioned medium of a murine leukemic T-cell line (STIL-3) as the source of interleukin 3 (IL-3). In soft agar gel, mast cell colonies were formed with a seeding efficiency of 100-200 per 10(5) bone marrow cells, which was much higher than previously reported data. Macrophages were distinguishable from mast cells by their characteristic "foamy" appearance in toluidine blue and May-Grünwald-Giemsa stained preparations and by their capacity to phagocytize carbon particles. Thus, the present method may provide a sensitive assay technique for the quantitative evaluation of mast cell progenitors.