Interleukin 6 is a multifunctional cytokine that exerts a variety of effects on different cell types. These effects include differentiation of B cells and cytotoxic T cells, growth promotion of hybridomas and activation of hepatocytes and mitogen-stimulated helper T cells. We identified and molecularly cloned a cDNA encoding a novel myeloid colony-stimulating activity from a human T cell line. This cytokine proved to be identical to the factor currently known as IL-6, thereby demonstrating effects of IL-6 with hematopoietic target cells. In addition to its ability to support murine granulocyte-macrophage colony formation, IL-6 was found to act synergistically with IL-3 in both the murine and human systems in support of colony formation by the primitive blast cell colony forming cell. This multitude of biologic activities suggests that IL-6 plays a prominent role within a network of cytokines in integrating the different arms of the host response to infection.