Abstract: Rabbit anti grass carp(Ctenopharyngodon idellus)brain serum(RACBS),rabbit anti Chinese soft shelled turtle(Trionyx sinensis)brain serum(RATBS)and rabbit anti Chinese soft shelled turtle(Trionyx sinensis)thymocyte serum(RATTS)were obtained. By complement dependent cytotoxicity assay and respective absorption assay of RATBS,RATTS with thymocyte,brain tissus of Chinese soft shelled turtle and grass carp, it is shown that there are Thy1 antigens(a brain associated thymus antigen or thymus brain antigen)on thymocytes and brain of Chinese soft shelled turtle,but absent in brain of grass carp. By indirect immuno peroxidase histochemistry staining,it is shown that the Thy1 antigens appear on the membrane of thymocytes and a parts of peripheral lymphocytes. Further,with indirect immunohistochemistry technique,CD4,CD8 cross reactions have been identified on lymphocyte membrane of Chinese soft shelled turtle using mouse monoclonal antibody directed against Human Leucocyte Differentiation Antigen CD4,CD8. In this paper,the persence and the significance of Thy1 antigen and CD,the specificity and the action of RACBS and RATTS,the hetergeneity of lymphocytes have been discussed.