Anesthesia and operation may impair the immune system so that bacterial growth and tumor cells spread can occur more rapidly and host response to transplanted tissue may be altered. In order to evaluate the influence of inhalation anesthetics on immune function, mitogen induced lymphocyte transformation and colony formation of T lymphoctye of peripheral blood in rats were studied. The experimental animals were divided into 4 groups according to inhaled anesthetics such as control, 0.8% halothane, 1.65% enflurane and 1.05% isoflurane 6 hours inhaled group. One day after inhalation of anesthetics, 5 ml of blood was sampled from inferior vena cava and the lymphocytes were isolated and cultured. Spontaneous and phytohemagglutinin (PHA) or pokeweed mitogen (PWM) induced lymphocte transformation were measured by the titration of H-thymidine uptake and the number of colony forming unit-T lymphocyte (CFU-TL) were counted. The results were as follows: Spontaneous lymphocyte transformation was increased by halothane and decreased by enflurane significantly but not differed by isoflurane compared with the control group. Lymphocyte transformation were decreased significantly before and after PHA stimulation in all of the anesthetic groups respectively compared with the control group. 3) Lymphocyte transformation by PWM stimulation also decreased in all of the anesthetic groups. 4) The numbers of CFU-TL cluster and colony decreased in all of the anesthetic groups compared with the control group. In conclusion, inhalation anesthetics such as halothane, enflurane and isoflurane decreased immune competence and that halothane was the most, isoflurane was the least immunosuppressive among these three inhalation anesthetics.