Plaque morphology of the HSV-1 (F), OncSyn and OncdSyn viruses. Nearly confluent Vero (a-c) and 4T1 (d-f) cell monolayers were infected with wild-type HSV-1(F) (a, d), OncSyn (b, e) and OncdSyn (c, f) viruses. Individual viral plaques were visualized 48 hr post infection by immunohistochemistry and photographed with a phase contrast microscope. Vero (g) and 4T1 (h) cells were infected with OncdSyn virus. Viral plaques were photographed 48 hr postinfection with a fluorescent microscope.