Poxvirus‐infected primary chick embryo fibroblasts have been used as a model system to study the mechanism of the antiviral activity of homologous interferon. The effect of pretreatment of cells with 10–300 units/ml of interferon on the synthesis of cowpox and vaccinia WR virus‐specific proteins was tested in the ‘early’ and ‘late’ phase of infection. [35S]Methionine‐labelled virus‐specific proteins were examined after separation from host cell proteins by sodium dodecylsulfate acrylamide gel electrophoresis followed by autoradiography. Synthesis of individual poxvirus‐specific proteins was affected to varying degrees by interferon treatment of the cells. Some ‘late’ viral proteins were synthesized at a reduced rate even if the cells had been treated with only 10 units/ml of interferon. On the other hand, even after pretreatment with high doses of interferon (1000 units/ml) synthesis of certain virus‐specific proteins could not be completely inhibited. In the late stage of infection with strain vaccinia WR or cowpox virus, a protective effect of interferon treatment on host cell protein synthesis could be detected. Vaccinia‐WR‐specific DNA synthesis is fairly resistant to interferon pretreatment, thereby making it possible to analyze the transcription program of virus‐specific RNA in the ‘late’ phase of infection in cells treated with low doses of interferon. All vaccinia‐WR‐specific RNA species transcribed in the ‘late’ phase of infection were detectable in chick embryo fibroblasts pretreated with 100 units ml of interferon which resulted in strong inhibition of the synthesis of several ‘late’ viral proteins. Cowpox‐virus‐infected primary chick embryo fibroblasts developed a distinct cytopathic effect at late stages of infection but vaccinia‐WR‐infected cells did not. Morphologically this cytopathic effect was reduced in interferon‐treated cowpox‐infected chick embryo fibroblasts.