FIG. 4.

Titration of extracellular virus with Vero cells. 143B cells were incubated with HSV-1 V701 for 7 h at 39°C and then at 20°C for 6 h. The 20°C block was reversed by incubation at 31°C for 24 h. The control cells were incubated for 30 h at 31°C without the 20°C block. The extracellular medium was collected and titrated on Vero cells for 4 days at 31°C.