Background: Localized mast cell accumulation has been reported in a number of human diseases. Suplatast tosilate has been reported to inhibit IL-3-dependent murine mast cell development, and therefore is expected to suppress mast cell growth proliferation at disease sites. However, there are differences between the growth factors required by human and mouse mast cells. Methods: Human mast cells were grown in serum-free culture condition from cord blood mononuclear cells with suplatast tosilate. Mast cell number was assessed from the cell numbers counted by a flow cytometer with constant flow rate and Kit+ percentage. Immunochemistry for tryptase was also examined on the cytocentri- fuged preparations. Results: Suplatast tosilate (100 μg ml ) dramatically inhibited human mast cell development from cord blood progenitors. On the other hand, the growth of 8-wk old mast cells that developed in serum-free media, and which reached over 95% in purity and still maintained proliferation ability, and the growth of the human mast cell line HMC-1 cells were not inhibited by suplatast tosilate. Conclusions: Suplatast tosilate may be useful for control of human mast cell numbers at the local sites by in- hibiting their development from their progenitors.