The differentiating leukemic cells can also be rescued from apoptosis by the tumor promoting phorbol ester 12-0-tetradecanoylphorbol 13-acetate (TPA) but not by the nonpromoting isomer 4-α-TPA (32). TPA rescues the differentiating cells from apoptosis by a different pathway than rescue with these cytokines. TPA can thus act as a tumor promoter by inhibiting apoptosis (32). It is therefore possible to activate the normal physiological process of apoptosis in myeloid leukemic cells, and this can be used to suppress leukemia even without induction of terminal differentiation. This has been shown by the decreased in vivo leukemogenicity of IL-6-pretreated leukemic cells with a viability factordependent state (83) and by the ability of antibody to IL-6 or to its receptor to suppress the development of multiple myeloma by IL-6-dependent myeloma cells (84, 85).