BALB/c mouse embryo cells morphologically transformed by papovavirus SV40 initially failed to produce tumors in immunocompetent or immunosuppressed syngeneic newborn or adult mice. Prolonged in vitro cultivation was required for the transformed cells to produce tumors in the immunosuppressed host. The level of expression of murine leukemia virus gs antigen in transformed cells did not increase until the transformed cells had undergone four or five in vivo passages suggesting that the malignant property of the transformed cells is expressed independently of the murine leukemia virus gs antigen.