Summary:Precursors of antigen‐specific cytotoxic T cells from mice pretreated with immunosuppressive doses of cyclophosphamide can be expandedin vitroandin vivoby low doses of human interleukin‐2. The effector cells are Thy 1+ and are not elicited in the absence of antigen. High doses of human interleukin‐2in vitrocan expand both normal and cyclophosphamide‐resistant T cell precursors into effector cells capable of lysing both natural killer cell‐sensitive and ‐insensitive target cells in the absence of antigen.