In atopic patients, allergen-sensitized T lymphocytes specifically proliferate in the presence of T cell-growth factor, interleukin 2 (IL-2). Lecithin-bound iodine (LBI), which has been used as a therapeutic modality for patients with bronchial asthma (BA), effectively inhibited Dermatophagoides farinae (Df) mite antigen-induced IL-2 responsiveness in concentrations comparable to LBI concentrations in blood. IL-2-responding T cells were more sensitive to LBI than antigen-presenting cells, whereas LBI suppressed the release of interleukin 1 (IL-1) elicited by Df antigen. In addition, ovalbumin (OVA)-induced IL-2 responsiveness in egg-sensitive patients and purified protein-derivative (PPD)-induced IL-2 responsiveness were similarly inhibited by LBI. The IL-2 responsiveness induced by concanavalin A (Con A), however, was not changed. On the basis of these results, LBI may act as a slight immunosuppressant to inhibit the induction of allergen-specific lymphocytes and to improve the clinical status in allergic diseases.