Enveloped viruses enter cells by binding to their entry receptors and fusing with the membrane at the cell surface or after trafficking through acidic endosomal compartments. Species-specific virus tropism is usually determined by these entry receptors. Because mouse mammary tumor virus (MMTV) is unable to infect Chinese hamster cells, we used phenotypic screening of the T31 mouse/hamster radiation hybrid panel to map the MMTV cell entry receptor gene and subsequently found that it is transferrin receptor 1. MMTV-resistant human cells that expressed mouse transferrin receptor 1 became susceptible to MMTV infection, and treatment of mouse cells with a monoclonal antibody that down-regulated cell surface expression of the receptor blocked infection. MMTV, like vesicular stomatitis virus, depended on acid pH for infection. MMTV may use transferrin receptor 1, a membrane protein that is endocytosed via clathrin-coated pits and traffics through the acidic endosomes, to rapidly get to a compartment where acid pH triggers the conformational changes in envelope protein required for membrane fusion.