The conditions and cell sources for colony stimulating activity (CSA) production by nasal polyp epithelial scrapings were examined. Epithelial scrapings removed from patients were grown to confluence during 7 days as monolayers of epithelial cells in media supplemented with fetal calf serum (FCS) on collagen-coated microwell plates. Growth kinetics of nasal polyp epithelial cells (NPECs) were determined, and CSA in NPEC conditioned medium (CM) was assessed with density-gradient separated, nonadherent peripheral blood mononuclear cells in standard 14-day methylcellulose assays. Nasal polyp cultures in the presence of 5% or 15% FCS (vol/vol) demonstrated significantly more epithelial cell proliferation than cultures at 0% and 1% FCS. There were comparable metachromatic cell counts in polyp epithelial scrappings from allergic and nonallergic donors. Similarly, NPEC CM from allergic and nonallergic donors had equivalent CSA for basophil/mast cell (BMC) and eosinophil (EO) lineages, respectively. CSA production was enhanced under conditions of higher FCS concentration and NPEC proliferation. These studies confirm an epithelial cell origin of BMC and EO growth and differentiation factors derived from nasal polyps and point to the existence of a unique microenvironment for BMC and EO development provided by polyp epithelium that appears to be independent of the presence of an allergic diathesis.