Abstract: 【Objective】To deeply understand the generation and development of coinfection of MDV and REV in broiler breeders,then find the optimal time of differential diagnosis.【Method】We studied pathohistological changes, apoptosis, immunohistochemistry and ultrastructure of tumor issues of broiler breeders inoculated with MDV and REV.【Result】The study showed that proliferation of small lymphocytes were seen in main organs at 1 week old, then immature lymphocytes,all kinds of lymphocytes,primitive reticular cells and Marek’s disease cells were observed after 2 weeks. Cell apoptosis couldn’t be seen untill 10 week in immune system. Immunohistochemistry detection showed that two special cells were observed in main organs at 2 weeks old. Multi-morphology Lymphocytes, MDV and REV, mitotic figures and apoptosis of lymphocytes were observed with transmission electron microscope.【Conclusion】Dynamic pathohistological study showed that pathological changes of coinfection chickens is earlier and manifester than single infection chickens. Coinfection chickens showed that inflammation is main phenomenon in early stage and tumour cells proliferated in late stage. So differential diagnosis can be done by pathohistology in early stage. Marek’s disease cells, primitive reticular cells, immature lymphocytes and two virus can serve as a basis for differential diagnosis. Immunohistochemistry　detection is accurate method for late stage.

Key words: Marek’s disease virus, Reticuloendotheliosis virus, coinfection, dynamic pathology, antigen location