Blood cell formation is governed by the haemopoietic growth factors that control the proliferation, maturation and survival of the haemopoietic progenitor cells via activation of receptors expressed on the cell membrane. Most of these receptors share structural features and have been grouped in the haemopoietin or class I receptor superfamily. Recently considerable progress has been made in elucidating the regions critical for the function of these receptors and the signal transduction mechanisms that they activate. Moreover, it has become clear that certain clinical haematological conditions can be linked to specific defects in these receptors. The significance of defects in the receptor for granulocyte colony-stimulating factor (G-CSF) in the pathogenesis of severe congenital neutropenia and acute myeloid leukaemias is discussed.