Influence of surgical intervention on the immunological status of 42 patients with various malignant neoplasm was evaluated and was compaired with that of 38 patients with nonneoplastic disease. Immune competency was measured with the percent ratio and population of circulating T-cell, which was identified based on the ability of the cell to form spontaneous rosette with sheep erythrocyte (E-rosette), and with the blastogenic activity of peripheral blood lymphocyte against phytohemagglutinin (PHA) and pokeweed mitogen (PWM). The delayed hypersensitivity was also tested by means of the tuberculin reaction. The result was summarized as follows, mainly on the basis of the data 10 days after the surgery. 1) The percent of T-cell (E-rosette forming cell) showed no distinctive difference as compaired with that in the preoperative period, although more than the half of the patient showed some tendency to increase in the ratio, and this trend was similar with that of the patient with nonneoplastic disease. However, the absolute number of the circulating T-cell was depressed in the patient with malignant neoplasm as well as in those with benign disease, and it was particularly remarkable in the aged of the former subject. 2) The most impressive difference between pre- and postoperative situation was observed in the blastogenic response of the peripheral lymphocyte to PHA in the cancer patient. The decrease in the response of the cell down to below the preoperative level was occurred in about two third of the patient with the average of 10 percent, whereas the response in the patient of nonneoplastic disease was rather raised above the normal level in about two third of them. Though no distinctive correlation was noted between the clinical stage of the cancer and the blastogenic response, the impairement of this activity was quite striking in gastric and lung cancers of the aged over 60 years. Furthermore, blood transfusion seemed to be against this impairement in the benign disease, but the data did not support that the similar effect of blood transfusion was working among the patients with malignant tumor. 3) The blastogenic response of the lymphocyte against PWM showed no uniform alterations following the surgery, both in the cancerous and noncancerous subjects, and the average reactivity was nearly equal to that in the preoperative condition. As for the cancer patient, the above response seemed to be not associated with difference in the disease, the age and the amount of transfused blood volume. 4) The incidence of negativation of the tuberculin reaction after the surgery was observed in 20 % of the cancer patient and in 14 % of the non-cancer patient. However, the available data were not sufficient to analize a correlation between the decreased blastogenic capacity of the lymphocyte in vitro and the lowered delayed hypersensitivity reaction in the skin, in association with the surgical intervention.