In embryo cell cultures in gel medium infected with Sindbis virus, two types of plaques appear: large (G) and small (p) plaques. G plaques appear within 24 hours, their size increases and their diameter reaches more than 20 mm; p plaques appear within 48 hours, and their diameter never reaches more than 3 mm. After three "clonages", stable p sublines and G sublines are obtained, the latter always yielding a constant percentage of p plaques.