Macrophage colony stimulating factor (CSF-1) is a member of a family of glycoproteins that are necessary for the normal proliferation and differentiation of myeloid progenitor cells. The human CSF-1 gene has previously been assigned to chromosome 5 using somatic cell hybrids, and further localized to 5q33 by in situ hybridization with a 3H labelled cDNA probe. However, the murine macrophage colony stimulating factor gene (csfm) has been localized to a region on mouse chromosome 3 which was previously shown to be syntenic with the proximal region of 1p and not 5q. Using a human genomic DNA clone that contains the CSF-1 gene, we have localized CSF-1 to chromosome 1p13-21 by fluorescence in situ hybridization. The reassignment of the CSF-1 gene argues against its involvement in myeloid disorders with deletions of the long arm of chromosome 5.