Objective:To evaluate the immunomodulating effect of oyster peptide on immunosup-pressed mice.Methods:ICR mice injected with cyclophosphamide (CTX)were adopted as the module group,with mice without treatment as the control group,and different dosages of oyster peptide (0.5 g/kg,1 .0 g/kg,and 2.0 g/kg)were given to the low,middle,and high groups for 1 5 days.The body weight,spleen,and thymus weight of the mice,structures under the microscope of the immune organs, numbers of white blood cells,ratios of T lymphocyte subsets,immune cytokines and numbers of nuclear cells,and DNA content in bone marrow were all assessed.Results:Compared with the control group, the structures of thymus and spleen of the mice in the CTX group appeared obscure and shrunk when ob-served under microscope,the number of their white blood cells declined (P =0.04),the proportion of their CD3 +T cells in peripheral blood declined (P =0.003),the proportion of their CD8 +T cells in pe-ripheral blood declined (P =0.002),the concentration of their IL-5 in peripheral blood significantly in-creased (P <0.01 ),the concentration of their nucleated cells and DNA density in bone marrow de-creased (P =0.04,P <0.01 ).Oyster could improve the structures of thymus and spleen of the immuno-suppressed mice.Compared with the CTX group,the number of white blood cells in 2.0 g/kg group in-creased (P =0.003),the proportion of CD3 +T cells in peripheral blood in 1 .0 g/kg group (P =0.04) and 2.0 g/kg group (P =0.02)increased,the proportion of CD8 +T cells in peripheral blood in 2.0 g/kg group increased (P =0.002),the concentration of IL-5 in peripheral blood in all the oyster treated groups increased (P <0.01 in 0.5 g/kg,1 .0 g/kg,and 2.0 g/kg groups),the concentration of IL-1 7 in peripheral blood in 2.0 g/kg group decreased (P =0.03),the concentration of nucleated cells in bone marrow of all the oyster treated groups increased (0.5 g/kg vs.CTX,P =0.04;1 .0 g/kg vs. CTX,P =0.02;2.0 g/kg vs.CTX P =0.01 ),the DNA content in bone marrow of all the oyster treated groups increased (P <0.01 in the 0.5 g/kg,1 .0 g/kg,and 2.0 g/kg groups).Conclusion:Oyster peptide could improve the structures of immune organs of the CTX-induced immunosuppressed mice,re-cover the imbalances of T lymphocyte subsets,improve the immune cytokines and increase numbers of nucleated cells and DNA content in bone marrow,thus improving the immunologic function.