Cellular turnover of the hematopoietic system is supported by a small population of cells termed hematopoietic stem cells. Stem cells are capable of self-renewal and differentiation into individual lymphomyeloid lineages. Available evidence indicates that the decision of a stem cell to self-renew or differentiate and the decision of a multipotential progenitor to select a lineage pathway during differentiation (commitment) are intrinsic to the progenitors and are stochastic in nature. In contrast, proliferative kinetics of the progenitors, namely, survival and expansion of the progenitors, appear to be controlled by a number of interacting cytokines. Whereas proliferation and maturation of committed progenitors are controlled by late-acting factors such as erythropoietin, macrophage colony-stimulating factor, granulocyte colony-stimulating factor, and interleukin-5, progenitors at earlier stages of development are controlled by a group of several overlapping cytokines. Interleukin-3, granulocyte/macrophage colony-stimulating factor, and interleukin-4 regulate proliferation of multipotential progenitors only after they are triggered to exit from dormancy state. Triggering of cycling of dormant primitive progenitors and proliferation of lymphohemopoietic primitive progenitors appear to require interactions of early acting cytokines including interleukin-6, granulocyte colony-stimulating factor, interleukin-11, interleukin-12, leukemia inhibitory factor, and steel factor.