day at about 2 u00d7 10 6 cells ml perfusion is increased to about two reactor volumes day and at about 4 u00d7 10 6 cells ml perfusion is increased to the maximal rate of 4 volumes day for 4 reactor volumes day will support a maximal density of more than 8 u00d7 10 6 cells ml the bioreactor harvests containing cells and viral particles may be isolated according to known methods retroviral particles are also isolated according to the invention initially bioreactor harvests containing cells and retroviral particles are clarified using a filter having about 0 5 to about 1 7 preferably about 0 7 to about 1 5 more preferably about 1 2 u03bc pore size to pass the retroviral particles and to retain cells and other debris the clarified harvests of infectious retroviral particles are pooled and well mixed to ensure homogeneity the infectious retroviral particles are then subjected to inactivation by adding beta propiolactone bpl at about 0 25 ml bpl l of supernatant with continues stirring followed by incubation the mixture for about 2 u00b0 c to about 8 u00b0 c preferably at about 4 u00b0 c to about 6 u00b0 c for about 18 to about 24 hours the pool is then raised to 37 u00b0 c and held at this temperature for about 2 to about 5 preferably about 3 to about 4 hours to hydrolyze residual bpl this step serves to chemically reduce infectivity of the virus by alkylating the various structural components such as.