Interleukin (IL)–10 induces a long–term antigen–specific anergy in human CD4+ T cells. Peripheral blood mononuclear cells (PBMCs) were isolated from house dust mite (Dermatophagoides farinae, Der f)–sensitized asthmatic patients. PBMCs were stimulated with Der f antigen for 72 h immediately after purification or after 48 h of resting culture with medium, and IL–10 and IL–5 in the culture supernatant were measured. PBMCs were also stimulated with Der f antigen for 7 days either immediately after purification or after 48 h of resting culture, after which cells were collected. Secondary proliferative responses of these cells to stimulation for 3 days with Der f antigen and mitomycin C–treated PBMCs as antigen–presenting cells or with phorbol myristate acetate (PMA) plus calcium ionophore were investigated. Stimulation of PBMCs with Der f antigen immediately after purification significantly increased the proliferative response and IL–5 production. Stimulation of PBMCs with Der f antigen after resting culture with medium alone for 48 h significantly decreased IL–5 production and markedly increased IL–10 production. Although activation of cells with Der f antigen immediately after purification significantly increased secondary proliferative responses, stimulation after 48 h of resting culture failed to increase secondary proliferative responses. However, proliferation recovered when cells were activated with PMA plus calcium ionophore. These results suggest that antigen–specific Th2 cells are anergized by IL–10 and that Th2 cell tolerance may suppress eosinophilic inflammation in allergic asthma.