Twenty-nine patients with colon cancer who underwent operations at the First Department of Surgery, Okayama University Medical School were studied. During operations, regional lymph nodes were collected and classified into four groups; groups N(1, 2, 3, and 4) Tumor grades followed Dukes classification. Cytotoxicity of the lymph node lymphocytes in each group and of the peripheral blood lymphocytes (PBL) in each patient was examined. Cell lines of P-4788 isolated from the metastaic ascites of the patients with colon cancer were used as target cells for examining cytotoxicity of the lymph nodes lymphocytes and PBL. As the lymph node lymphocytes and PBL were closer to cancer lesions, their cytotoxicity tended to decrease and was significantly different between N(1 , 2, and 3) groups and N(4) group and between N(1, 2, and 3) groups and PBL group (p<0.01). As the cancer progressed, the cytotoxicity decreased and was significantly different between Dukes A, B, and C groups and Dukes D group p<0.01). Compared between groups of negative and positive metastatic lymph nodes, the cytotoxicity of lymph node lymphocytes and PBL tended to decrease in the positive group, but it was not significantly different. When the tumor of 8 cm in diameter was made to be border for tumor classification, cytotoxicity of PBL and regional lymph node lymphocytes decreased as the diameter of the tumor became greater and significantly different (p<0.01).