The fate of parental SV40 DNA in monkey kidney cells was investigated by infecting the cells with purified virus labelled with [3-H]-thymidine. 5-Bromodeoxyuridine (200 mug/ml) was added to the cells at 2 h after infection to label virus progeny DNA. At 72 h post-infection the cells were harvested and virus DNA was extracted and fractionated by isopycnic sedimentation in CsCL solution. The following DNAs with characteristic densities were found: light (LL), 1.70 g/ml; hybrid (HL), 1-75 g/ml and heavy DNA (HH), 1-80 g/ml. Of the total cell-associated [3-H]-radioactivity (derived from parental virus), more than 90% was recovered in unreplicated parental DNA(LL DNA), about 2% was recovered in the HL DNA and about 1-7% was associated with the HH DNA. The unreplicated parental DNA was present as uncoated intact DNA complexed with proteins present in the infected cell. The HL DNA contains one light parental and one heavy progeny DNA strand. The nature of the radioactivity present in the HH DNA remains to be determined.