Two different oncogenic viruses, polyoma and SV40, are capable of transforming mouse cell line 3T3. The properties of the transformed cells produced by the two viruses are in some ways similar, but in other ways they are specific for the infecting virus. This fact permits testing whether a cell line transformed by the one oncogenic virus is still susceptible to the transforming action of the second virus. Two different clonally isolated polyoma-transformed lines when infected with SV40 give rise to cells with properties characteristic of SV40-transformed cells. The frequency of transformation, however, is considerably reduced compared to that of the parent cell line, 3T3.