Abstract: To reveal the immunosuppression mechanisms,eight-day-old Muscovy ducklings,were challenged with Muscovy duck reovirus(MDRV)and avian influenza virus(H9AIV)individually and simultaneously.The changes of proliferation of B lymphocytes in the blood,spleens and bursa were determined by MTT method 7 days,14 days and 21 days after the Muscovy ducklings were challenged.The results show that B lymphocyte stimulation indexes in the blood,bursa and spleen of the challenged group were significantly lower than that in control group,and B lymphocytes stimulation index in co-infection group was the lowest.So B lymphocytes proliferation of peripheral blood and humoral immune organs(bursas,spleens)in Muscovy ducklings was inhibited by both MDRV and H9AIV,and co-infection with MDRV and H9AIV could aggravate this immunosuppression.