Precursors of antigen-specific cytotoxic T cells from mice pretreated with immunosuppressive doses of cyclophosphamide can be expanded in vitro and in vivo by 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-2 in vitro can 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.