Transfer of the human hypoxanthine phosphoribosyltransferase (HPRT) gene into the murine myeloma cells in vitro was accomplished with retrovirus-derived vectors. To construct expression plasmids for the human HPRT gene, human HPRT cDNA was ligated into murine retroviral vector, pRSVneo vector conferring resistance to neomycin-kanamycin (Tn5) antibiotics and viral long terminal repeats. Vectors of subcloned and modified from it were introduced into cultured HPRT deficient murine myeloma cell (SP2/0) by calcium phosphate-mediated DNA transfection. The transformed cells were selected against G418 or hypoxanthine/aminopterin/thymidine(HAT) medium and were used for HPRT enzyme activity. The HPRT activity in the lysates of the transformed cells were increased more ten-fold than that of HPRT-deficient SP2/0 cells. we now investigate whether these transformed cells contain authentic human HPRT or not.