Proliferation and differentiation of hematopoietic cells are controlled by pleiotropic regulatory molecules. While the sequences of these factors are not related, their membrane receptors are restricted to two gene families with homologous domains. The members of the hematopoietic (or cytokine) receptor family (for erythropoietin, interleukins-2, -3, -4, -6 andu-7, granulocyte-macrophage and granulocyte colony-stimulating factor) are composed of multiple subunits necessary for high-affinity binding and cell signalling. Signal transducing mechanisms are largely unknown. The occurrence of variant signal transducers in different tissues could explain the pleiotropy of these regulatory molecules. Members of the receptor tyrosine kinase family bind dimeric forms of macrophage colony-stimulating factor, stem cell factor and platelet-derived growth factor leading to kinase activation and phosphorylation of many substrates involved in production of second messengers. Soluble forms (binding proteins) exist for members of both families. These may be proteolytic cleavage products of transmembrane receptors or naturally secreted products. Such binding proteins can potentially function as inhibitors in feedback regulation and in protection and transport of cytokines and would provide a rational therapy when cytokines are produced in excess. Knowledge of signal transduction mechanisms and of the three-dimensional structure of ligands and receptors can lead to the design of drugs with cell-specific effects.