It has been demonstrated that a 15 l microcarrier culture can produce an equivalent amount of virus as that achieved from 700 × 850 cm 2 roller bottles despite having the surface area of only 400 roller bottles. This maintenance of the cell productivity has not always been observed when virus production systems have been scaled up. Griffiths et al. (1982) had a tenfold decrease in cell productivity of HSV per cell when switching from roller bottle cultures to a microcarrier system using Vero cells. This may have been an artefact of poor virus release from the cells. In comparison, when culturing the HSV-2 on MRC-5 cells, a significant but smaller decrease in production levels was observed when changing from roller bottle cultures to microcarrier cultures. This observation reinforces the issues made in some early virus production work reported by Giard et al., 1977. They proposed that there are specific virus/cell line requirements coupled with a specific optimisation of the conditions not only for the growth of the cells but also for the virus production phase. The production of hepatitis A in microcarrier culture was reported to be 8-fold lower than when cultured in conventional flask cultures (Widell et al., 1984) . The maintenance of productivity seen in this study is very encouraging in moving the culture system forward to a larger scale which may be suitable for the production of Phase III material and for the supply of commercial requirements. Due to the productivity's shown here, production at a 15 l scale would be large enough to supply material for Phase I and Phase II studies.