To improve the purity of lentiviral vector supernatants for clinical studies we have evaluated plasmid DNA removal from lentiviral vectors and also the extent of plasmid DNA associated with transduced CD34 cells in an ex vivo transduction protocol. Optimal conditions of plasmid DNA removal by benzonase treatment were established by varying the temperature, time, and benzonase concentrations in the reaction mix and were determined to be 50 units of benzonase per milliliter of vector supernatant at 37 degrees C, for 15 min. No plasmid DNA was detected, suggesting efficient plasmid degradation was achieved under these experimental conditions. The infectious titer of benzonase-treated lentiviral vector (RRL-CMV-GFP) was nearly identical to the titer of untreated vector (2.3 +/- 0.3 x 10(6) transduction units per milliliter (TU/ml) and 2.7 +/- 0.3 x 10(6) TU/ml, respectively). Analysis of plasmid DNA in concentrated lentiviral vectors shows that concentration substantially decreases the amount of DNA per TU. Analysis of the extent of plasmid DNA associated with transduced CD34 cells in an ex vivo transduction protocol suggests that a minimal amount of plasmid is transferred to transduced cells if the vector supernatant was not previously treated with benzonase. In conclusion, benzonase treatment is effective in eliminating plasmid DNA from vector supernatants and treatment does not affect infectious titers. However, because there is minimal transfer of plasmid DNA to transduced cells under ex vivo transduction conditions, DNA removal from lentiviral vectors may not be essential for all ex vivo clinical applications.