T-lymphocytes, B-lymphocytes and macrophages in vivo maintain orderly constant immunological adjustment mechanisms by acting with each other in a complex manner. T-lymphocytes, that are main actors of cell-mediated immunity and form the rosette by binding with sheep red blood cells, are by no means a single cell group, and along with functional differentiation they show peculiar markers on the cell surface so that they can be divided into individual subsets. In view of the fact that an early foreknowledge or diagnosis of immunological behaviors, especially the acute rejection, in the patient transplanted with kidney, would not only lessen disturbances of the transplanted kidney but also it would diminish the danger of subsequent complication, time-lapse measurements were taken frequently of total T-lymphocytes and active T-lymphocytes known to have biological activity using peripheral blood before and after kidney transplantation, and their changes were studied. The subjects consisted of 22 living related and 9 cadaveric kidney transplantations that we handled in the First Department of Surgery, Okayama University during the period from March 1974 to August 1978. Peripheral blood lymphocytes were isolated by Ficoll-Conray method (a modification of Boyum's method) from 5ml of fresh heparinized blood obtained from kidney-transplanted patients. By the method of Tachibana-Yata the sheep red blood cell suspension was prepared for measuring total T, active T, and B-lymphocytes, and the measurements were taken by the test-tube method of Felsburg et al, -Kerman et al. To total T and active T lymphocytes the nonspecific rosette formation reaction with sheep red blood cells was applied, and to B-lymphocytes the complement binding reaction with complementsensitized sheep red blood cells was used. The reaction of lymphocytes to sheep red blood cells was conducted in the ratio of about 1:100, and those sheep red blood cells showing the adhesion of over 3 sheep red blood cells were taken as the rosette formation positive cells. The lymphocyte subpopulation was measured frequently for a long period of time before and after the operation of kidney transplanted patients, and the percentage of total T (Tt), active T (Ta) and B-lymphocytes was calculated, and such a percentage was observed along with changes of clinical symptoms after the kidney transplantation, and these findings were used to serve for the early fore-knowledge of acute rejection crisis or lung infection, and also to serve for the determination of the optimal dose of immuno-suppressive agents. The results are described as follows. 1) The test results by the test-tube method with 36 materials from 20 normal individuals are: Tt 60.3±6.1% , Ta 40.2±10.3% , B 21.0±8.0% . The results of 18 cases undergoing hemodialysis prove to be Tt 52.3±8.8% (p<0.01), Ta 32.5±10.6% (p<0.02), and B 25.0±9.7% . 2) In those cases showing favorable progress after kidney transplantation there can be observed a marked decrease in Tt and Ta ratios, which gradually show a recovery tendency after 3 months, but the decrease of T-lymphocyte level continues for a fairly long period of time. Especially the decrease of Ta level is markedly prolonged. 3) Four to 9 days before the onset of acute renal rejection Tt and Ta showed a significant rise, but by MPSS-high-dosage treatment there was a rapid decreasing tendency. 4) In those cases having lung infection the decrease of T-lymphocyte level has been continued for a fairly long time before the onset of disease, especially marked is the decrease of Ta level. Those having favorable results after treatment showed a significant rise of T-lymphocyte level, showing a recovery tendency. 5) Ta ratio can be obtained by a simple procedure and the result can be clarified in a short period of time, and it is a useful immunological parameter that reflects sharply and accurately the immunity of the host. 6) From these findings it is concluded that the ratio of Tt and Ta lymphocytes, by measuring frequently along with lapse of time would give the foreknowledge of postoperative immunosuppressive conditions and acute rejection crisis in kidney-transplanted patients and the prognosis determination of lung infection cases. In addition, such a ratio is useful in determination of the optimal dose of immuno-suppressive agents.