It is worth illustrating a morphologic technique for the study of cells in productive and resistant infections. We have used surface replica technique, which provides a simple means for looking at the outer surface of cells as they grow on coverslips. Figure 8A.1 illustrates Vero cells in productive SSPE infection. Only the infected cells have surfaces that seem to be covered with granular strands. These strands appear 2 days after infection and begin to increase in numbers and thickness until they eventually become more convoluted. Immunoperoxidase method, in which reaction products of the peroxidase can be visualized on the surface replica, allows recognition of viral antigenic sites.