Leukocytapheresis (LCP) for the treatment of patients with diseases that involve an abnormal autoimmune reaction aims to improve the condition of the patient's pathology and to correct imbalances in immunological regulation mechanisms by removing the responsible leukocytes from the peripheral blood. To clarify the mechanism of therapeutic effect, LCP was conducted in healthy volunteers to investigate changes in peripheral blood leukocyte and platelet counts over time during the treatment. The subjects were 10 healthy male volunteers. LCP was performed once in each volunteer for 3,000 ml of blood volume. The peripheral blood counts decreased significantly, reaching a minimum of 20.0% of the baseline number of leukocytes, 10.1% of the baseline number of neutrophils, and 40.3% of the baseline number of lymphocytes. The number of removed leukocytes was about 6.6 x 10(9) cells, including about 3.5 x 10(9) neutrophils, as well as about 5.0 x 10(11) platelets. After the completion of LCP, the peripheral leukocyte levels increased transiently (overshoot), and at 2 h after the completion of the treatment, they reached 193.4% of the baseline value. Since LCP is capable of reducing the peripheral blood leukocyte count over a short period of time, its impact on peripheral blood is great. In addition, in view of the overshoot phenomenon and the appearance of immature granulocytes, the LCP may affect not only the peripheral blood, but also the bone marrow pool, the marginal pool, and the leukocytes present in the tissues.