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Marked cytopathic changes were induced by challenge with Newcastle disease (ND) virus in bovine testicle or kidney cell cultures which were previously infected with non‐cytopathogenic strains of bovine diarrhea (BD) virus. No cytopathic changes were induced by ND virus in similar cells not infected with BD virus. The development of cytopathic effect was shown to be associated with enhancement of ND virus replication. This exalting effect of BD virus appears to be dependent on infectivity, since the effect was inhibited when infection of the cells with BD virus was blocked by specific antiserum. Various factors involved in the phenomenon were investigated and an in vitro method (END) for the assay of BD virus and its antibodies was developed. The use of this method eliminates the difficulties in recognizing non‐cytopathogenic strains of BD virus which hampered systematic investigations of the nature and behavior of BD virus as well as of the natural history and pathogenesis of the infection in cattle.