In secondary calf testis cell cultures, infection with both RP and BVD viruses at the same time (mixed infection) resulted in cytopath ic effect (CPE) which was mainly related to RP virus. The cytopath ic effect related to IBR predominated after its use with either RP or BVD in mixed infection. No. blocking effect was observed for BVD virus in clutures infected with this virus followed after sometime by RP virus (superinfection). The virus of IBR was severe ly destructive, and the cultures superinfected with this virus followed either with RP virus or BVD virus did not surive long. However, the cytopathic effects in cultures infected firstly with BVD virus followed by IBR virus was slightly delayed. The results were explain ed on the basis of interferon production, and their importance for vaccine production, mainly RP vaccine, was discussed.