The effects of mitomycin C and 60Co gamma-irradiation, which induce production of SV40 from SV40-transformed hamster cells, on the replication of superinfecting SV40 or virus DNA in cells varying in permissivity for SV40 replication have been examined. These agents enhance replication of SV40 in an uninducible line of SV40-transformed hamster kidney cells and in nonpermissive secondary hamster kidney cells. The same treatments do not affect SV40 replication in semipermissive hamster (BHK21) and human (HEL, HEK) cells and inhibit SV40 replication in permissive monkey (TC-7) cells. We conclude that forms of induction treatment, such as mitomycin C or 60Co gamma-irradiation, modify the expression of host cell factors which determine the level of permissivity for SV40 infection.