In the present study, we report for the first time in Egypt the evidence for variants of avian leucosis virus subgroup J. ALV-J associated with 2 cases of mixed infection with Marek's disease virus, MDV (one case is associated with tumor and the other from homogenates of blood and tissues of chicks experienced transient paralysis syndrome of MDV) was detected. The ALV-Js might be variants as indicated by histopathological behavior; negativity of the ALV-J specific PCR; and serological profile in the flock with mixed infection (tumor-cases of ALV and MDV) and in experimentally infected chickens with blood and tissue homogenates of other mixed infection case of ALV-J and MDV was detected. Using specific PCR for REV and ALV groups A or B or C and/ or D, all samples of both mixed infection cases revealed negative results for amplification. However, 28.3% of sera samples collected from chickens with tumor-cases at age of 38 weeks were positive for ALV-J and negative for ALV (groups A and B) by ELISA. Testing of sera collected from the flock with tumor-cases at 45 and 55 weeks of age revealed that 38.2% and 20% of sera were positive for ALV (group A and B), respectively. In experimentally-infected chickens with homogenates of blood and spleen, 14.2% of sera were positive to ALV-J and negative for ALV (group A and B) when tested at 30 weeks post-inoculation. Histopathological examination revealed the occurrence of mixed infection of MDV and ALV-J with unique pathological lesions eye and liver which were found to have heavily aggregations of myelocytes characteristic to ALV subgroup J.