This chapter summarizes the literature dealing with T cell-derived lymphokines that regulate hematopoietic/lymphoid lineages. Lyinphokines individually regulate the proliferation and differentiation of a number of lineages of cells. One of the first T cell-derived lymphokines to be extensively characterized was interleukin 2 (IL-2). This growth factor was detected by its ability to induce the proliferation of T cells in vitro. To determine the mechanisms by which lymphokines such as IL-3, IL-2, or granulocyte/macrophage colony-stimulating factor (GM-CSF) mediate their effects in differentiation and proliferation will involve effort. The integral relationship of various cellular homologs of oncogenes to growth regulation has provided an extraordinary means to identify genes, which are involved in the regulation of cell proliferation. Recent studies have suggested that factors exist that stimulate the proliferation of activated or nonactivated B cells, induce the differentiation of B cells or both. In addition to the factors affecting T cells and B cells, lymphokines are produced by activated T cells that induce the proliferation and differentiation of hematopoietic cells. The dynamic characteristics of the response to the question that is much more characteristic of hematopoietic systems and early lymphoid lineages and deals with the mechanisms involved in differentiation are most evident in the response of normal bone marrow cells to IL-3. In this response, there is an obligatory sequence of changes in gene expression.