When tested on RK(13) cell cultures, strains of rubella virus could be differentiated by their ability to form small or large plaques. Large plaques were produced by the HPV-77 and Cendehill strains, and also by a laboratory stock strain (West Point), after only 14 passages in RK(13) culture. Five wild-type rubella viruses, isolated and passaged only a few times in African green monkey kidney tissue culture, grew well in RK(13) cell culture, but they were sensitive to agar inhibitors and, therefore, formed small plaques. On the other hand, RA27/3, an attenuated strain grown in WI-38 human fibroblast cells, developed low titers in RK(13) cells and also produced small plaques. We concluded that the morphological differences between small-plaque and large-plaque viruses depended on their sensitivity to agar inhibitors and on the pH of the medium during plaque formation.