It has been demonstrated that the activation and proliferation of T lymphocytes is a crucial link in enhancing immune function by low dose ionizing radiation. Transferrin receptor (TfR) is one of the important membrane receptors expressed in T cells during activation and proliferation of T lymphocytes and is closely related with the activation and proliferation of T lymphocytes. TfR expression in lymphocytes in mouse spleens was examined using direct immunoflurescence and flow cytometric analysis. The results showed that whole body irradiation (WBI) with low dose X —rays could stimulate TfR expression in lymphocytes in mouse spleens, and this might be another evidence for enhancing immune function with low dose radiation and meanwhile indicated that TfRs play an important role in enhancing immune function with low dose radiation.