The relationship between tumor cells and tumor-infiltrating lymphocytes (TIL) in tumor tissues from 12 cases of carcinoma of various sites was studied at the electron microscope level. Only at few TIL got in touch with tumor cells by several patterns of contact. Some tumor cells contacting with lymphocytes showed destructive changes similar to that of apoptosis. These findings indicate that some TIL can kill tumor cells in vivo and the main mode of tumor cell death is apoptosis, However, there were only a few TIL in contact with tumor cells. the tumoricidal effect of TIL in tumor tissue may be very limited.