SV40 minichromosomes (MCH) either isolated from SV40 infected CV-I monkey cells (native MCH) or reconstituted in vitro from viral DNA and the H1 depleted calf thymus histone fraction could transform and mutate Chinese hamster (CH) cells in vitro. Whereas reconstituted MCH transformed and mutated CH cells with about the same efficiency as purified SV40 DNA, approximately 10-200-fold increase in the transforming activity had been demonstrated for native MCH. All transformed cell colonies and a major part of the isolated mutant cell clones recovered after inoculation of CH cells with SV40 MHC expressed the SV40 T antigen. Addition of H1 to both purified SV40 DNA and reconstituted MHC drastically diminished the transforming capacities of both agents. Possible reason(s) for the inhibition effect of H1 histone is discussed.