Blood cell formation is continuous in adult life and requires complex regulatory control to achieve the necessary cell proliferation, differentiation commitment and maturation. Much of this control is achieved by specific glycoprotein haemopoietic regulators, more than 20 of which have now been identified and produced in recombinant form. For the granulocyte-macrophage lineage, major regulators are the colony stimulating factors (CSF) that are produced by many cell types dispersed throughout the body. The production of CSF is rapidly increased in response to induction signals, most usually of microbial origin, representing a demand-generated signalling system designed to activate and generate cells to rapidly eliminate the micro-organisms. The CSF not only control cell division but certain aspects of differentiation commitment, maturation initiation and functional activation. These multiple responses are initiated by signals arising from different regions of the cytoplasmic domains of the CSF membrane receptors. Less well understood are the signalling systems controlling the special gene activation events required to initiate the formation of the first haemopoietic cells and the specialized micro-environmental cells needed to support haemopoietic cells as they respond to haemopoietic regulators.