The four major glycoprotein colony stimulating factors (CSF's) controlling murine granulocyte-macrophage formation and function have been purified to homogeneity and some sequence data obtained. cDNA's for GM-CSF and Multi-CSF have been cloned and the full sequence of the CSF polypeptides deduced. Despite functional similarities between the two molecules, no sequence homology exists between these two regulators. Binding studies using radiolabeling M-CSF, G-CSF and GM-CSF have indicated that relatively low numbers (200-700) of specific receptors exist for G-CSF and GM-CSF on responding hemopoietic cells although up to 20,000 receptors per cell exist for M-CSF. Despite the few receptors available, biological effects of the CSF's are demonstrable with low receptor occupancy. Interactions between the four CSF's permit the production of granulocytes and macrophages to be finely controlled.