IL-2 production and IL-2 receptor (Tac antigen) of the peripheral blood mononuclear cells in 30 patients with aplastic anemic (AA) were studied. We found that mononuclear cells from patients produce spontaneously IL-2 in the absence of exogenous lectin stimulation, the proportion of Tac+ cells in mononuclear cells increased. The release of IL-2 and/or Tac antigen expression were elevated in almost every patient with AA. The plasma from patients stimulate mitogen-induced blastogenesis and Tac antigen expression of normal human lymphocytes. Immunological 1 abnormalities of patients with AA possibly might represents secondary response to bone marrow depression.