When inoculated at the same MOI, Vero cells released a larger amount of infectious rubella virus into the culture medium than did BHK21 cells. However, BHK21 cells (in monolayer or in suspension) produced more intracellular infectious virus than Vero cells when tested 24 h after infection. Maturation of the virus in BHK21 cells occurred at the plasma membrane and, in a larger quantity, in the cytoplasm (Golgi apparatus and vacuoles). Viral particles consisted of an electron-dense core (32 nm) surrounded by a capsid and enveloped by a single membrane (8–10 nm). Aberrant forms (elongated and twisted) in the vacuole and double virions in the plasma membrane were observed as early as 65 h after infection.