Mononuclear cells (tumor-infiltrating lymphocytes [TIL]) that invade tumor tissue in fresh surgical specimens were examined and characterized. Such cells were seen in virtually all 52 tumor specimens analyzed and were found to be physically attached to the tumor cells. The morphology of these cells is described with scanning and transmission electron microscopy. Phenotype analysis of these tumor-infiltrating lymphocytes was also performed. In the fresh surgical specimens, while the tumor cells are viable, TIL cells are mostly null cells (except for weak positivity for Leu-1 antigen). As the culture ages in vitro, the TIL phenotype changes drastically to a wide variety of both T and B cells as well as monocyte-macrophage lineage. While these phenotype changes occur, the tumor cells to which the TIL are attached display progressive signs of necrosis and cell death. Tumor cells without attached TIL cells are not affected by necrotic changes. It is speculated that these described changes in the phenotype and morphology of TIL may be related to acquiring cytotoxic activity against the tumor cells. An attempt to control these changes may possibly be useful for clinical application in cancer immunotherapy.