This study was designated to detect reticuloendotheliosis virus (REV) as a contaminant in fowl pox vaccines. A total of 30 fowl pox vaccine samples were examined for the presence of REV using the in vitro and in vivo methods. In the in vitro test, the fowl pox vaccine samples were inoculated into chicken embryo fibroblast (CEF) cultures prepared from SPF embryonated chicken eggs and examined by PCR test for detection of REV. In the in vivo test, each fowl pox vaccine sample was inoculated in five days old SPF chicks and kept under observation up to 12 weeks post inoculation (PI); serum samples were collected at 15^th, 30^th, and 45^th days PI for the detection of antibodies against REV by commercial ELISA kit, and tissue samples were collected at 8^th and 12^th weeks PI for histopathological examination. Results revealed that: only one imported vaccine sample gave positive results by PCR test a product of 291- bp was obtained by the vaccine sample. Serum samples collected from positive vaccine batch were tested for REV antibodies by ELISA test and the sera were positive. Histopathological examination for liver, spleen and bursa of fabricius revealed the presence of tumor cells in the examined organs and these changes confirmed the results obtained by PCR and ELISA tests and indicated that the sample is contaminated with REV. The data clearly indicated that screening of all commercial poultry vaccines to be virus free is an important factor in assuring the biosafety of animal vaccines.