The hemopoietic growth factors interleukin-3 (IL-3, multi-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) belong to a family of secreted glycoproteins that stimulate the proliferation and differentiation of hemopoietic progenitor cells. IL-3 and GM-CSF have overlapping biological activities and show similar regulation of expression after mitogenic or antigenic stimulation of T lymphocytes. In the present work we have derived a map of the region covering the Il-3 and Csfgm loci using a combination of pulsed-field gel electrophoresis and molecular cloning. The two genes are shown to be 14 kbp apart, in the same orientation with the IL-3 gene 5' of the GM-CSF gene. The proximity of the two genes, together with similarities in their structure, function, and regulation, suggests that they may have arisen by ancient gene duplication.