A reliable technique has been devised for the preparation of colorectal tumour-infiltrating lymphocytes (TIL). The immune capacity of these lymphocytes has been assessed in vitro and compared with that of lymphocytes infiltrating the lamina propria of adjacent normal mucosa (LPL) and with autologous peripheral blood lymphocytes (PBL). Assay of a natural killer (NK) cell function revealed the absence of such activity in TIL and LPL depsite the presence of normal levels in PBL. Antibody-dependent (K cell) cytotoxic activity was also absent in TIL and LPL. Both TIL and LPL showed significant mitogen-induced cytotoxic responses, although higher levels were detected in PBL. Tumour-infiltrating lymphocytes revealed depressed levels of spontaneous DNA synthesis, but mitogen stimulation of TIL was equivalent to that of LPL. T-cell proportions in TIL preparations were equivalent to those in PBL, but LPL comprised significantly fewer T cells.