Under defined plaquing conditions, three attenuated strains of rubella virus showed different plaque morphology and plaque size when tested in Vero cell cultures. The Cendehill strain of rubella virus formed distinct, ring-shaped plaques, while strains HPV77 and RA27/3 produced clear plaques that could be distinguished by their different size. These distinguishing characteristics remained preserved even after the vaccine strains were passaged five consecutive times in Vero cell cultures.