We investigated the effects of Dermatophagoides farinae (Df) and interleukin (IL)-2 on the release of eosinophil colony-stimulating factor (Eo-CSF) activity from mononuclear cells (MNC) and lymphocytes of patients with bronchial asthma (BA) who were sensitive to Df to clarify its relationship with IL-5 and granulocyte/macrophage colony-stimulating factor (GM-CSF). MNC and T cells of patients cultured with IL-2 and Df released Eo-CSF activity. These Eo-CSF activities were partially inhibited by anti-IL-5 and anti-GM-CSF antibodies. In 11 of 15 cases studied, MNC from patients produced GM-CSF in response to IL-2. In four of 15 cases studied, MNC from patients produced GM-CSF in response to Df. On culture with IL-2 or Df, the releases of IL-5 into the medium by MNC from individual patients varied. The results indicate that in BA responsiveness of lymphocytes to Df is increased, and suggest that IL-5 and GM-CSF produced by T cells play a role in the induction of eosinophilia and the pathogenesis of BA.