The two murine haemopoietic growth factors, granulocyte-macrophage colony stimulating factor (GM-CSF) and Multi-CSF (interleukin 3) stimulate the proliferation and differentiation of an overlapping set of haemopoietic progenitor cells and are produced coordinately following activation of T lymphocytes. Here we report the chromosomal location of the genes encoding these two factors. Initially both genes were assigned to chromosome 11 by analysis of mouse/Chinese hamster somatic cell hybrids. Genetic analysis using an interspecies (Mus musculus X Mus spretus) back-cross confirmed this assignment by demonstrating that both the GM-CSF and Multi-CSF genes are genetically linked to the SPARC gene, which had been independently assigned to sub-band B1 of chromosome 11. Analysis of physical distances by pulsed field gel electrophoresis demonstrated further that the two CSF genes lie within 230 kb of each other. However examination of the subchromosomal region containing all three loci by pulsed field gel analysis showed that SPARC is at least 400-500 kb distant from the region containing the two CSF genes.