Infected cells (Figure 4) were harvested from the microcarriers 60-72 h post infection when the cells on the majority of the microcarriers showed 100% CPE. Harvesting was accomplished by removal of the media from the culture, addition of hypotonic saline at a volume equal to 10 ml hypotonic saline per roller bottle equivalent of surface area of microcarriers used in the culture. The detached cells were separated from Cytodex 1 by filtration through a sterilisable 70µm mesh in the bottom of the vessel. The resulting cell/virus supernatant was subjected to low pressure disruption (Degouys et al., 1997) , (Figure 5) , to release the DISC-HSV virus from the cells and cellular debris. Virus titres were calculated using an in-house TCID 50 assay with subsequent conversion to pfu ml −1 .