Marek's disease virus (MDV) is an oncogenic alphaherpesvirus. Its specific phosphorylated protein, pp38 has been implicated in MDV oncogenesis. In order to check whether the known anti-viral or anti-proliferative actions of interferon (IFN) are of importance in Marek's disease (MD), chicken embryo fibroblasts (CEFs) were infected with attenuated serotype-1 MDV strain CVI988, or with herpesvirus of turkeys (HVT). Different concentrations of native chicken IFN were added to the cell cultures, prior to their infection. After incubation, MDV plaques were counted. Analysis by flow cytometry for pp38 expression was performed by using three monoclonal antibodies (MAbs) and for HVT by using an anti-glycoprotein B (gB) MAb. Increasing IFN quantities caused a reduction in a stepwise manner of plaque numbers as well as a suppression of pp38 and gB expression in the CVI988- and HVT-infected cells, respectively.