The results described in this paper indicate that the integration of the SV40 genome into human chromosome 7 results in the transformation of the human cells and in the expression of SV40-induced antigens. If integration of the SV40 genome in human chromosomes other than 7 ever occurs, it does not result in cell transformation and the expression of the virus-induced antigens. The expression of the SV40 T antigen in different monkey cells transformed by an adeno 7-SV40 hybrid is also related to a specific monkey chromosome. Somatic cell hybrids between normal nondividing mouse cells and SV40-transformed human cells behave as transformed cells and contain, without exception, the human chromosome 7 carrying the SV40 genome. Since no segregation into SV40 T antigen-negative hybrid clones was observed in these hybrids, it is inferred that the presence of the human chromosome 7 carrying the SV40 genome in the hybrids is mandatory for cell division.