Subpopulations of the infiltrating lymphocytes in breast cancer tissue from 31 patients were identified by indirect immunoperoxidase technique with antihuman T- and B-cell sera. In all noncancerous lesions examined (seven cases), B-cells were predominant and T-cells were scarcely found. In contrast, T-cells were predominant in breast cancer tissues (17 in 21 cases). T-cells tended to contact closely with cancer cells or cancer cell nests and accumulated around and in the walls of venules draining the cancer, while B-cells tended to cluster focally apart from cancer cell nests. T-cell infiltration was scanty in scirrhus carcinoma, whereas it was ample in infiltrating papillotubular carcinoma which had a better prognosis. There was a significant reverse correlation between the intensity of the T-cell infiltration and the clinical stages. The intensity of the T cell infiltration was significantly high in patients without lymph node metastasis. These facts suggest the possibility that the infiltrating T-cells in cancer tissue represent host resistance against cancer and that the intensity of the T cell infiltration correlates with the clinical prognosis of the breast cancer patients.