SUMMARY Intrinsic interference is not limited to Newcastle disease virus: vesicular stoma- titis (VSV) and orthomyxoviruses can be similarly inhibited at the stage of RNA synthesis. Detailed investigations of the interference of fowl plague virus (FPV) by VSV show: (I) adsorption of the interfering virus is not responsible; (2) the synthesis of all FPV virus components is affected, if the interfering virus is present during the initial stages of FPV replication and (3) competition of the interfering RNA polymerase with heterologous templates can be excluded by experiments carried out in vitro.