The novel stromal cell factor, IL-11, has been reported to have diverse effects on the lymphopoietic and myeloid/erythroid cells in vitro. These include expansion of T cell-dependent Ig-secreting B cells, proliferation and differentiation of megakaryocytic progenitors and of a variety of myeloid and erythroid precursor cells, and multiplication of pluripotential hematopoietic progenitors. In addition, IL-11 inhibits adipogenesis in vitro. In vivo administration of IL-11 elevated the number of circulating neutrophils and platelets and increased the number of megakaryocytes in the spleens of normal mice.