Eosinophilia is a feature of nasal polyposis. The aim of this study was to determine the role of cytokines and allergen in maintaining the eosinophilic infiltrate in this condition. Polyp fragments from house dust mite (HDM)-sensitive atopic individuals and nonatopic individuals were cultured in the presence of HDM, or phytohaemagglutinin (PHA) or culture medium alone. Culture supernatants were assayed for interleukins (IL) 3, 5, and 8 and granulocyte macrophage colony stimulating factor (GM-CSF), and eosinophil survival enhancing activity (ESEA) in vitro. Significant ESEA was produced spontaneously. When polyp tissue from atopics, but not from nonatopics, was stimulated with allergen for 2 days there was a further increase in ESEA associated with a median 12 and fourfold increase in IL-8 and GM-CSF, respectively. This increased ESEA was markedly reduced with anti-GM-CSF and, to a lesser extent, anti-IL-8 blocking antibodies. When stimulated with PHA, polyp tissue from atopic subjects also produced increased ESEA, implicating possible T-cell involvement. This was associated with a small (twofold), but significant, increase in IL-8 and a less consistent increase in GM-CSF. However, anti-IL-8 or anti-GM-CSF blocking antibodies failed to reduce the ESEA in these supernatants, suggesting involvement of other mechanisms. This study suggests that in sensitized individuals, allergen may contribute to polyp eosinophilia by stimulating the production of granulocyte/macrophage colony stimulating factor and interleukin 8.