Granulocyte/macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic growth factor which has been shown to induce proliferation and activation of inflammatory cells, and may play a role in allergic diseases and experimental allergic reactions. Since little is known about the involvement of cytokines in allergic inflammation in the lung, we investigated whether human lung fragments produce GM-CSF in vitro. The present studies demonstrate that human lung fragments produce GM-CSF in vitro and that glucocorticoids are potent inhibitors of this cytokine production. Human lung was cut into fragments, rinsed, and cultured in 60-mm tissue culture plates containing 50 mg of tissue in RPMI 1640 with antibiotics in the presence or absence of a variety of steroids for 18 h. Lung fragments were rinsed and then incubated for an additional 4 h. Supernatants were harvested and analyzed for GM-CSF activity using the GM-CSF/interleukin (IL)-3 responsive M-07e human luekemic cell line. Steroids alone had no effect on M-07e proliferation. Human lung fragments produced 32.1 + 11.8 ng of GM-CSF equivalents per gram wet weight of tissue during the 4 h incubation (mean _+ S.E.M., n = 5, range 9.2-74.2). While specific antisera against human GM-CSF neutralized 96.8 _+ 2.8% (n = 5) of the activity, anti-IL-3 antibody had no effect, suggesting most or all of this activity was GM-CSF. Treatment of lung fragments in vitro for 18 h with hydrocortisone (HC) inhibited the production of GM-CSF dose-dependently. Maximal inhibition of GM-CSF production was 72.8 + 4.0% at a concentration of l0 -6 M hydrocortisone (n = 5), and the molar concentration of HC that inhibited of GM-CSF production by lung tissue by 50% (ICs0) was approximately 4.5 x 10 -7 M. Kinetic studies revealed that a 6 h preincubation with the drug was required for 50% inhibition of GM-CSF production. HC and other glucocorticoids, at