Cell-mediated immunity in chronic myelogenous leukemia (CML) was investigated by means of three kinds of skin tests, including purified protein derivated of tuberculin (PPD), candida and dinitrochrolobenzene (DNCB), and macrophage migration inhibitory test (MIT). Immunological examinations were made on 21 patients with CML, who have been admitted to our hospital or controled in our outpatient department since October 1974, at the same, 20 healthy adults were used as control study. The following results were obtained: Ⅰ. Skin tests: 1) Thirteen (61.9%) of 21 patients showed positive reactions to the PPD at least once throughout clinical course of each patient, while positive in 18 (90.0%) of 20 controls. 2) Candida test was attempted on a total of 13 patients. One (7.7%) of these 13 patients showed positive reaction to the candida, while positive in 9 (45.0%) of 20 healthy subjects. 3) Seven (41.2%) of 17 patients showed positive reactions to the DNCB sensitization, while all of 20 controls proved to be positive. 4) The results of PPD skin test were analyzed according to the stages in clinical course of each individual. Positive reactions were obtained from 50% of the patients at pretreatment stage, 66.6% at remission stage, 20% at relapse stage and 16.6% at blastic crisis. 5) The PPD skin test changed from negative, or false positive, to positive in 5 of 21 cases who had achieved remission during the observation period, while it changed from positive to negative in one case who had relapsed during the same period. It kept positive in 4 of 8 cases who had maintained remission, and kept negative in one relapsing case having failed to achieve remission. In 2 of 6 cases who had a blastic crisis during the observation period, it was positive before the crisis and became false positive after the crisis. According to these results a significant correlation was found between PPD and clinical stage of CML. Ⅱ. Macrophage migration inhibitory test (MIT): 1) In the preliminary studies, the optimal ratio was determined that 10(6) lymphocytes react properly to 0.1ml antigen fluid, which is equivalent to 0.11-0.03μ g of protein level and is extracted from the leukemic cells by means of sonication. 2) Eight (53.3%) of 17 patients were regarded as positive in allo-antigen system of MIT and 5 (55.5%) of 9 patients in auto-antigen system. Because of the difficulty of collecting lymphocytes, all MIT were conducted only in the remission stage of each patient. From these results, the following conclusion can be made. Cellular immunity in CML is somewhat impaired at pretreatment or relapsing stage. It is, however, repaired mostly at remission stage and it has a close correlation to clinical stage of CML.