A number of studm have indicated that IL-6 exerts antitumoral activity in vim, in some leukemias and against metastasing tumors (see refs. 1 and 2 for reviews). These studies in murine models would support applications of IL-6 therapy in specific clinical settings, such as prevention of metastases following resections of primary tumors or prevention of relapses in certain types of leukemias. However, such clinical studies have not yet materidized, due to the limited understanding of how IL-6 works on various cells, why it stimulates growth of some cells (eg. myeloma and other hematopoietic cells) while inhibiting others, and due to the limited understandmg of the mechanisms underlying the antitumor effects observed. This article reviews what we have learned from studies on murine models of leukemias and metastases and from in vitro studies on a gene regulatory effect of IL-6 that may be relevant to growth control in many cell types.