Hematopoietic stem cells are capable of self-replication and differentiation to lineage-committed progenitor cells. The progenitors proliferate and differentiate to lineage-specific, morphologically recognizable precursors and, finally, to terminal circulating blood cells. These homeostatic mechanisms are regulated by a complex set of interacting growth stimulatory and inhibitory factors that are produced by, or in collaboration with, the tissue's regulatory microenvironment. A number of well-characterized cytokines have been implicated in the negative regulation of hematopoiesis: ferritin H-subunit (HF), lactoferrin (Lf), prostaglandin E (PGE), tumor necrosis factor (TNF), interferon (IFN), transforming growth factor-beta (TGF beta), acetyl-N-Ser-Asp-Lys-Pro (AcSDKP) or thymosin-beta 4, pyroGlu-Glu-Asp-Cys-Lys (pEEDCK), macrophage inflammatory protein-1 alpha (MIP-1 alpha), inhibin, superoxide dismutase (SOD), glutathione (GSH) and others not well-known yet. The role of inhibitors in restraining stem cells from entering the cell cycle and protecting them from the toxic side effects of chemotherapeutic drugs is opening an alternate strategy for the treatment of cancer patients.