The purpose of this study was to evaluate the effects of high-intensity endurancetraining (HIET) on immunological response (IR). Twenty-four (24) long distanceathletes (age= 18.8±3.1 yrs, ht=176.3±3.3 cm , wt=62.9±4.9 Kg , VO2max=59.6±3.3 ml/kg/min) were matched and then randomly assigned to a high intensitytraining group (HIT, n=12) or control group (n=12). Both groups ran for 1 hour pe y for consecutive days. The HIET group ran at 75﹪VO2max and the controlgroup ran at 60﹪HRmax. On day one, three and seven blood samples were collectedbefore, immediately after running and 1 hour during recovery. The changes inphytohemagglutinin (PHA) and pokeweed mitogen (PWM) which stimulated lymphocyteproliferation were determined. The plasma complements C3c and C4, C reactiveprotein (CRP), creakine kinase (CK), and immunoglobulin A (IgA), G (IgG), M(IgM), were tested. The whole blood of natural killer cells ( NK cells ), T helper cells (TH cells), T cytotoxic cells(TC cells), B cells, and the activities of cytokine IL-2, IL-2 receptor (IL-2R) and IL-4 were also measured.The results indicated that the titer of (PHA) and (PWM) stimulated lymphocyteproliferration, and the concentration of NK cells, TH cells and TC cells significantly decreased (p<.05), at 1 hour of recovery. The concentrations ofCK were within the normal range immediately after and 1 hour after running. The complements C3c and C4, and CRP did not change after exercise. It is concluded that 1 hour of HIET will not cause tissue damage and inflammatory response.But HIET can temporarily decrease immune function, and the decrease is accumulative. This may increase runners'' infection rates due to a decrease in lymphocyte proliferation, concentration of NK cells, TH cells and TC cells .