Two kinds of RNA capable of producing interferon in a heterologous cell system occur in [chick embryo] cells producing interferon in response to induction by Newcastle disease virus. One type has messenger activity for interferon and is a 10-22 S single-stranded cellular mRNA. The other can induce interferon production by heterogenous cells, and is resistant to treatment with RNase and has a sedimentation constant > 35 S. This RNA is possibly primary replicative RNA of the virus-inducer.