We studied the correlation of a microcytotoxicity assay (MCA) and a 3H-Uridine postlabeling lymphocytoxicity assay (TPLA). Cells of the P-4788 cell line established from the pleuritis carcinomatosa of a colon cancer patient were used as the target cells. MCA correlated well with TPLA. The cytotoxicity index in the case of P-4788 cells as the target cells was high in comparison with the cytotoxicity index when M-Hela cells, a cell line from a uterine carcinoma, were used. A mixed lymphocyte tumor reaction with allo-antigen extracted from colon cancer was done, and the stimulation index of colon cancer patients was found to be high in comparison with that of gastric cancer patients and benign disease patients. There was no significant difference between the cytotoxicity of each stage of cancer before operation. However, there were significant differences the cytotoxicity before that after operation in all stages except stage V. There was a tendency for the blocking index to increase with the advance of cancer. Serum carcinoembryonic antigen (CEA) values increased with the advance of cancer and decreased after operation in all stages. Serum CEA values correlated with the blocking index, but not with the cytotoxicity index.