Transmissible retroviruses encoding human hypoxanthine phosphoribosyltransferase (HPRT) were used to infect mouse bone marrow cells in vitro, and the infected cells were transplanted into mice. Both active human HPRT-protein and chronic HPRT-virus production were detected in hematopoietic tissue of the mice, showing transfer of the gene. These results indicate the possible use of retroviruses for somatic cell therapy.