Intracellular signaling events occurring downstream of receptor activation for the colony-stimulating factors GM-CSF and G-CSF and Steel factor the latter a member of the tyrosine kinase receptor family of hematopoietic growth factors, are discussed. Hematopoietic signaling pathways, including the Ras/Raf-1/MAP kinase cascade and the Jak-STAT pathway are defined and links existing between separate signaling pathways are discussed. Emphasis is given to exploring the relationships that exist between activation of receptor-associated proteins and signal transduction pathways, and the regulation of gene transcription, translation, and hematopoietic cell proliferation. A model system exploring the synergistic interaction between GM-CSF and Steel factor in the regulation of hematopoietic cell proliferation is presented.