Measles virus normally causes disease in human beings, and the host range of this virus may be determined by a specific receptor on the surface of primate cells. Human-rodent somatic cell hybrids were tested for their ability to bind measles virus, and only cells that contained human chromosome 1 were capable of binding virus. A study of lymphocyte markers suggested that the complement regulator known either as membrane cofactor protein or CD46 was the measles virus receptor. We proved this hypothesis by demonstrating that hamster cell lines that expressed human CD46 could subsequently bind virus. Furthermore, infected CD46+ cells produced syncytia and viral proteins. Finally, polyclonal antisera against CD46 inhibited virus binding and infection. These results prove that human CD46 permits cells both to bind measles virus and to support infection.