In the agar diffusion test 24 triazines were investigated with regard to their action on the multiplication of DNA phages (λ and LPP‐1) and RNA phages (M 12 and Qβ). In several cases the amount of plaques was diminished or increased depending on the kind of triazine and virus. The investigations demonstrate the triazines to be able to interfere with the formation of plaques by virulent and temperate viruses of procaryotes.