Lymphocytes prepared from the blood, tumour-draining lymph node and tumour were tested for immune reactivity with freshly isolated autologous tumour cells from biopsies. Reactivity was assessed by the autologous tumour stimulation assay and by lymphocytotoxicity. Activity was found in 6/11 blood preparations, 7/10 lymph nodes and 1/7 tumour-infiltrating lymphocytes in tumour stimulation assays and in 6/19 blood, 8/18 lymph-node and 5/20 tumour-infiltrating lymphocytes in cytotoxicity assays. Tests with material from patients with nasopharyngeal carcinoma showed a higher frequency of cytotoxicity in the tumour-infiltrating lymphocytes than in other solid tumours. There was a correlation between results of the two assays when performed on the same preparations and between the levels of reactivity in lymph node and blood from the same patient. Cytotoxicity in the lymph nodes showed specificity in that cells from a long-term culture (K562) known to be sensitive to natural killer activity and from allogeneic tumour biopsies were only rarely damaged. Cytotoxicity against K562 was more frequently determined in blood lymphocytes. Tumour-infiltrating lymphocytes were non-reactive in patients when the blood and lymph-node lymphocytes were active.