Our previous work demonstrated that under the conditions of restraint stress and under the control of central nervous system (CNS), an immune suppressive protein of stress (ISPS) was generated in peripheral lymph tissue and released into the blood stream, acting as an immune suppressor. In the present work, a protein similar to ISPS was found in human tonsil (a peripheral lymph tissue). Human tonsil was homogenized and the extract was prepared. It was found that lymphocyte proliferation was significantly suppressed by the extract. The suppression induced by the extract was partially reversed by the monoclonal antibody against ISPS (2C4). In ELISA test, the extract was able to bind to the monoclonal antibody. By immunohistochemistry, many ISPS positive cells were found in human tonsil. The ISPS positive cells were also found in human lymph nodes. Taken together, all the results demonstrate that a protein similar to ISPS may exist in human peripheral lymphoid tissue.