It is generally recognized that immunosuppressive manifestations such as lymphopenia, granulocytosis, decreased transformation to various antigens and impaired neutrophil chemotaxis are followed by surgical operation. In connection with this, anesthetics were also targetted as the possible depressive agent in the immune mechanism. To elucidate the theories, the present study was carried out with patients who were subjected to surgical operation utilizing anesthesia by halothane inhalation. Lymphocytes, specially B-and T-cell lymphocytes were examine throughout the course from operation to 48 hours after the operation. In general, T-cells were decreased by operation with halothane anesthesia, and decrases of Helper-T cell and Inducing-T cell were prominent than suppressor T-cell or Killer T-cell. However, the decrases showed recovery from six hours after the operation, and reached to normal values in two days. The overall result in the study suggest that anesthesia utilizing halothane may bring potentially harmful effect on the immunologic responses to infectious diseases or malignant, but not systemic.