A growth factor for normal and neoplastic mouse mammary epithelial cells in primary culture was partially purified from the conditioned medium (CM) of mouse mammary fibroblasts. The N-terminal amino acid sequence of the major 33 kD protein separated on SDS-polyacrylamide gel electrophoresis under reducing conditions suggested that the growth factor is closely related to hepatocyte growth factor (HGF). To examine whether mammary growth factor is identical to HGF we cloned mouse HGF cDNA from mammary fibroblasts using PCR with primers designed from the previously reported human and rat HGF cDNA sequences. We then expressed the cloned mouse HGF cDNA in COS-1 cells. The CM from mouse HGF cDNA-transfected COS-1 cells stimulated the growth of mammary tumor cells in a manner similar to the CM from mammary fibroblasts. From these observations it is concluded that mammary growth factor produced by mammary fibroblasts is identical to mouse HGF.