Autologous mixed lymphocyte reaction (AMLR) is an important reaction regulating the immune system in the various diseases. This is the first report investigating AMLR not only in peripheral blood but also in the spleen of gastric cancer patients. AMLR in the spleen of gastric cancer patients was significantly suppressed compared with that of peripheral blood of gastric cancer patients and patients without cancer. In investigation of AMLR composed of peripheral blood cells and spleen cells of gastric cancer patient, AMLR on splenic non-T cells as a stimulator was significantly suppressed compared with peripheral blood non-T cells as a stimulator. It suggests that abnormalities of non-T cells caused suppression of AMLR in the spleen of gastric cancer patients.