Summary The excellent growth of rubella virus in BHK21 cells is further enhanced by a number of substances. DEAE-dextran increased the plaquing efficiency, yield of virus, and cytopathic effect of the virus, particularly when present during virus adsorption. The incorporation of semicarbazide into maintenance media improved plaquing efficiency and cytopathic effect, but not virus yield. Other substances such as actinomycin and 5 fluoro-2'deoxyuridine, when present in low dosages in the maintenance media of infected cells, increased virus yield, cytopathic effect, and plaquing efficiency.