The haemopoietic stem cell occupies a central position in the hierarchy of the haemopoietic system and it is at this cellular level that all haemopoietic function can be ultimately regulated. Much efforts has thus gone into characterizing regulators of stem cell proliferation with a view to enhancing our understanding of the regulation of this important cell, and in addition to examining the potential clinical roles of such stem cell active factors. We focus on inhibitors of haemopoietic stem cell proliferation and review their molecular and cellular biology and potential clinical usefulness in cancer therapy. The potential roles of inhibitory molecules in the pathogenesis of leukaemias are also discussed.