Retrovirus coated with VSV-G may be concentrated using multiple rounds of centrifugation. A. Assessment by flow cytometry of transduction by retrovirus following concentration using different numbers of rounds of centrifugation. 1 μL of retrovirus was added for each transduction. B. Titration of concentrated viral stocks. Bars denote the mean viral titer ± standard deviation. Diamonds represent the fold change in viral titer. The trendline shows a linear relationship between the fold change in viral titer and the number of rounds of centrifugation. C. Addition of an addition round of centrifugation without addition of unconcentrated supernatant does not result in a decrease in viral titre. D. Demonstration by flow cytometry of successful transduction of primary mouse bone marrow cells by retroviral particles concentrated using multiple rounds of centrifugation. E. Viral titers rapidly decrease following storage of virus at 4 degrees C for 7 days. F. Time course of viral titers obtained following four rounds of centrifugation of supernatant collected on the given day post-induction (removal of antibiotics/tetracycline). 5 μL of retrovirus was added for each titration.