An increased number of histochemically distinct mast cells and eosinophils can be observed in the tumor mass of nasal inverted papilloma. In order to elucidate the mechanism of this accumulation of mast cells and eosinophils in a local site, we examined the activity of conditioned medium (CM) in inducing differentiation and proliferation of basophilic cells and eosinophils from hematopoietic progenitors of peripheral blood mononuclear cells of the patients with allergic diseases. CM was obtained by the culture of nasal inverted papilloma cells (NIPC) and its activity was measured by determining colony proliferation in methylcellulose assay for 14 days. In the 14 day methylcellulose assay, the number of Eo-type colonies in the presence of 1, 5, 10 and 20% CM was significantly higher than the baseline number of Eo-type colonies (0%). Cytochemical analysis confirmed that the number of colonies of cells with metachromatic granules and histamine positive colonies in Eo-type colonies in the presence of 5, 10 and 20% NIPC-CM was significantly higher than that in the control. The number of ECP positive colonies in Eo-type colonies in the presence of 5% and 10% CM was also significantly higher than in the presence of 0.5% CM. Furthermore both histamine and ECP positive colonies were observed in Eo-type colonies. These results are consistent with the hypothesis that basophilic cells and eosinophils accumulate at nasal inverted papilloma sites at least in part by the recruitment of progenitors from the circulation and subsequent differentiation in situ in response to cytokines. Eosinophils and basophilic cells also commonly originate from committed granulocyte progenitors.