Purified SV 40 DNA and SV 40 minichromosomes, either isolated from SV 40 infected cells (NaMi) or reconstituted from viral DNA and the H1 depleted calf thymus histone fraction (ReMi), were tested for their ability to transform primary Chinese hamster lung (CHL) cells. Using the agar suspension technique as the transformation assay, an approximately 200 fold increase in transformation activity for NaMi in comparison with the SV 40 DNA and an approximately 50 fold increased activity in comparison with ReMi could be demonstrated. Uptake experiments suggest that NaMi are taken up quicker and more efficiently from the CHL cells than ReMi or purified DNA. The transforming activity is most significantly using the calcium technique and the infected cells in higher passage numbers. Transformed colonies recovered after infection of CHL cells with SV 40 DNA, NaMi or ReMi revealed SV 40 related T-antigen in 90-100% of the cells tested. The number of T-antigen positive cells was stable over a period of 80 passages. This observation suggest a stable state of transformation by SV 40 DNA and SV 40 minichromosomes.