The initial steps in the DNA-transfer, or transfection, method of virus rescue were characterized using primary green monkey (GMK) cells exposed to SV40-transformed mouse (SV-3T3) cell DNA in the presence of 1 mg/ml DEAE-dextran. When large amounts (10-50mug) of high molecular weight DNA (greater than 10(7) daltons) were inoculated onto 10(6) GMK cells, usually less than 1 mug became cell-associated. DNA fragmented to a size of 1-3 X 10(6) daltons was bound more efficiently by the recipient cells, but generally only 5-10 per cent of the inoculum (representing 1-4 mug) was taken up. Approximately 50 per cent of the cell-associated DNA had penetrated to a DNase-resistant state by the end of the 30-minute incubation period. The effect of the size of thr transformed cell DNA molecule on the recovery of SV40 in transfection experiments was investigated. The trend appeared to be that rescue was more efficient with the larger molecular weight samples.