The in vitro production of granulocyte/macrophage colony-stimulating factor (GM-CSF) by mononuclear cells (MNC) from the peripheral blood of patients with bronchial asthma (BA) was examined by enzyme-linked immunosorbent assay (ELISA). GM-CSF concentrations in the media of MNC from patients with BA cultured with interleukin-2 (IL-2) was 80.2 +/- 52.0 pg/ml (mean +/- SE, n = 12), and that in cultures without a stimulant was 12.1 +/- 11.3 pg/ml. The GM-CSF concentrations in the media of MNC from patients with other diseases (n = 13) and from healthy volunteers (n = 6) cultured with or without IL-2 were less than 7.5 pg/ml (the minimum detectable value). The culture media of MNC from patients with BA demonstrated activities for stimulating the proliferation and survival of eosinophils, and these activities were partially inhibited by anti-GM-CSF antibodies. GM-CSF production by MNC of patients with BA treated with glucocorticoids was lower than that of MNC from untreated patients with BA, and it was inhibited by coculture with glucocorticoids in vitro. These results suggest that GM-CSF production by MNC is increased in patients with BA, is modulated by glucocorticoids, and may play an important role in the pathogenesis of BA.